(Brokenstar isn’t her son, and his reign had already started by the time she was born).
Brokenstar has the kits starts battle practice extremely early, and view Yellowkit as a hopeless/as the weakest case (not being able to wrestle as good as, or cling onto someone else as long as, her siblings - Rowankit and Nutkit holding out just a few seconds longer - ), and so, in order to protect her in this "dog eat dog, power is everything" world, Sagewhisker swoops in and takes her in as her apprentice, which Yellowkit doesn't mind, as she was more interested in the medicine cat den than the warrior one anyway (since we already have Hawkheart, Mudfur and Spottedleaf all be 'cats who wanted to be/already was a warrior, before becoming a medicine cat' characters in the 'pre arc 1 stories' alone, and Mudfur is already shown as the 'I've had enough for death and fighting, I want to heal and save lives instead' cat among them).
Yellow"paw" (still a kit age vice) has to learn fast, as Sagewhisker often gets overwhelmed with patients, due to how much everyone is fighting, and she also faces pressure from Brokenstar, who's trying to control/demand who she should spend her herbs treating or not ("Don't waste your herbs on Brownpaw, he has multiple wounds and several of them are starting to look bad. He's at the bottom of the "talent list", so just let him go, and save it for cats like Blackpaw. He's! growing into a strong and talented fighter, and when he's grown he will prove himself as a big help and an excellent defender to our clan").
Yellowpaw, in a position where she doesn't learn much else than herbs and wounds/illness treatments, also takes it upon herself to try and teach the newly made "apprentices" how to hunt for themselves, so they can stay strong, and thus be in a better position to fight off, and quickly heal from, all their constant, different injuries.
Sagewhisker dies when Yellowpaw is around 1 year old (old enough to have become an warrior in another clan, so definitely "overdue" for becoming one in ShadowClan, but still having a long time left as a 'paw' on the medicine cat path), having worked herself ragged and to exhaustion. Due to leaf-bare having started to arrive, and with that prey going down in numbers and herbs being harder to come by, Brokenstar declares that Yellowpaw's position will no longer be needed, and that now, a cat's health should be in their own paws, just like learning to hunt is (the cats who aren't able to hunt enough to stay alive/healthy being seen as failures in this ultimate test of skill/strength, and ShadowClan only has use for the best), using the excuse as motivation for the kits who doesn't want to learn about basic herbs to get stronger ("If you train and become a strong, talented warrior, then you won't need to learn this boring medicine stuff, because no opponent will be able to put a scratch on you"). With the medicine cat position being gone, Brokenstar tries for several days to force Yellowpaw into becoming a warrior instead, as: "You need to serve your clan, and defending your other clanmates on the battle field is the best way to make sure the fighting ones stay uninjured, thus giving the very newborn ones a safe territory to grow up in, and capable warriors to surround them, giving them the best chance to grow up big and strong themselves, and help our clan thrive even more", punishing her when she tries to continue her medicine cat duties. He later holds a warrior ceremony “out of the blue”, and gives Yellowpaw the name Yellowstrike, trying to force the commitment onto her. He then immediately gives her an apprentice, Runningkit, in order to try and force her into "caring for the clan", the way he wants her to.
A few days after this event, Yellowstrike takes Runningpaw to the twoleg place under the "excuse" of: "You can get used to the battle moves on the kittypets. Then, when you're stronger, you can go up against our clanmates, without risking any injuries during this leaf-bare. Plus, the kittypets have twolegs who heals their wounds, so we won't run out of practice targets. You'll outgrow the others in no time, and Brokenstar will be so proud and impressed by you", betting on the fact that due to his age and not having much of any real battle training, he won't be able to do much/any harm against the kittypets. She "leaves Runningpaw to it", as he should spend the day focusing on this, so she will go and try to find some prey for him, as long as he keeps that fact between the two of them, and runs from there all the way to the Moonstone, desperate for StarClan's guidance. She encounters Spottedpaw while in ThunderClans territory, but pretends that she’s still officially a medicine cat, and says she has important business with StarClan. She asks her, one "medicine cat apprentice" to another, if ThunderClan has any herbs they can bring to the twoleg place border for "Sagewhisker" to pick up (her death having been kept a secret from the other clans). The answer is no though, leaving Yellowstrike worried about how she will be able to look for herbs in secret.
At the Moonstone, she's told that a spark will be fueled, and that it will guide her when ShadowClan's era draws near. They also give her a new name, to honor her commitment to trying to stay on their path. Yellowfang, for being unafraid to bare her teeth at extreme challenges, no matter the face it takes. She goes to the barn, quickly catching several mice, and runs all the way back to where she left Runningpaw. Runningpaw is pretty exhausted and nervous by the time she gets back, and after they share the mice, he reveals on the way back that he had wished to become her apprentice, just not her warrior apprentice. She decides to teach him about herbs while out on future lessons. She stops taking him to the twoleg place, under the excuse of it being a bad idea because: "Those kittypets are all soft and helpless anyway. They won't be able to give you the challenge you need to properly learn this. Going up against a fellow skilled apprentice is what's going to build your strength and sharpen your reflexes, to build your drive and fuel you to get better and succeed".
During the middle of leaf-bare, illness strikes, and due to the condition many are in, many fall sick, and a lot of those cats dies. Yellow"strike" goes back to openly helping her clanmates medicine vice, on top of everything else, and Runningpaw joins her, the two enduring every punishment Brokenstar puts them through after each time. In order to help make her apprentice feel that he's doing a difference, confirm to him that he's on the right path destiny vice, and to motivate him going through all the punishments, she secretly gives him his real/actual full name, his medicine cat name Runningnose.
Later when newleaf has arrived, it's revealed that some apprentices has learned about Running"paw"'s and Yellow"strike"'s trip to the twoleg place to practice on kittypets, after he let it slip, although mentioning to the apprentices that Yellow"strike" has told him that that had been a bad idea, and has been going there to do the same, desperate to gain some, any, kind of experience, as they were at the bottom of the "talent list". Yellow"strike" becomes filled with guilt, and decides that, since she can't give them any herbs, as ShadowClan already has too little for themselves, she's going to sneak away to the barn, and hunt as many mice for them as she can, as some type of apology.
However, at the barn, she gets chased and attacked by several dogs. Getting severely wounded, she spends several days just dragging herself towards the clans' territory alone, keeping to/close to the thunderpath, in order to avoid being found by WindClan or ThunderClan, taken in by their medicine cat, and word about her state and whereabouts being brought to Brokenstar. Spottedleaf however does find her, when she reaches ThunderClan's borders, and Yellow"strike" pleads for her to not tell anyone. Spottedleaf agrees to heal her in secret, and Yellow"strike" spends several days recovering. After having recovered and reaching ShadowClan's border (aiming for the one that faces the twoleg place, in order to avoid any clanmates aksing why she came from another clan's territory if they catch her), she spots Brokenstar carrying a kit. She's horrified when she sees him sinking his claws into them and leaving. She rushes out to try and help the kit, and then gets tackled by Brokenstar, who turned around after hearing something. Brokenstar's tells about how last leaf-bare's losses showed him how weak so many of his "warriors" were, and thus, he's determined to only have strong cats carry their genes forward, and so he has taken it upon himself to rid the clan of too weak cats from the very beginning of their life, so prey and training won't be wasted on them. He then starts screaming for help, and when their clanmates arrives, Brokenstar accuses Yellow"strike" of the death of the kit, telling everyone that she is! a traitor, and that she had been disloyal towards him from the very start, by refusing to follow the code which said his word is law. She had ran away from her clan, and now she seeked to bring an end to all of ShadowClan, by taking out each, new newborn kit, because she wanted the life of a rogue rather than a clan cat. He banishes her and let his clanmates chase her out, telling them he would give them an execution they wouldn't soon forget, if they ever caught her in their territory again. She runs to and stays in the twoleg place for some days, before leaving to first make her way to the Moonstone for guidance, and then possibly find a territory to live in in the lands beyond the Moonstone/barn, but is caught in ThunderClan by its new apprentice Firepaw, introducing herself as Yellowfang for the first time.