Maybe it's a curse. Some evil magic might have been cast upon her, turning her into a cat against her will.
It could be a genetic mutation in a fictional, magical sense. Her family might have some long - hidden secret gene that allows such a transformation when certain conditions are met.
Well, one possible reason could be peer pressure. If she gets involved with a bad crowd that promotes certain behaviors, she might start to change. For example, if her new friends are all into excessive partying and dressing provocatively, she could be influenced.
In some stories, a scientific experiment gone wrong could lead to a boy transforming into a girl. For example, if a scientist was experimenting with genetic alteration and something unexpected happened. Another possibility is a wish granted by a mischievous or well - meaning supernatural being. Maybe the boy made a wish without fully understanding the consequences and ended up being transformed into a girl.
Magic could be one reason. Like in many fantasy stories, a magical spell or a enchanted object can cause the transformation. For example, a magic amulet that when touched by a boy, starts the process of turning him into a girl.
Well, it might be due to a powerful magical artifact. She could have accidentally touched or used an enchanted item that had the power to turn humans into animals, like a cursed amulet. Or, in a more spiritual sense, it could be a connection to a spirit world where cats are guardians and she has been chosen to take on that form for a special mission. There could also be a deal made with a supernatural entity that she has to fulfill, and the transformation is part of the deal.
Another reason might be related to some kind of spiritual or mythical connection. In certain mythologies, there are beings that can transform between human and animal forms. She could be related to such a being, and due to some trigger, like reaching a certain age or entering a specific place, she turns into a cat.
In some stories, the forest or a special place with strong natural magic can be the cause. The girl might unknowingly step into a place where the magic is so concentrated that it changes her form. This could be a place that has been untouched for centuries and has its own will to transform those who enter it into something more in tune with nature, like a cat.
Well, in such stories, a common theme could be punishment. If a girl has done something wrong in the story, like being disobedient or greedy, her transformation into a cow could be a form of punishment. It could also be about transformation and change in general. The girl going from a human to a cow represents a huge shift, and it could be used to explore how people adapt to big changes in their lives. And perhaps, in some cases, it could be a theme of sacrifice. The girl - turned - cow might be sacrificed for the good of the community in the story.
Magic could be one reason. Maybe she angered a sorcerer or a witch who then turned her into a cat as a punishment. Another reason could be a curse from an ancient object. If she touched a cursed amulet or something similar, it could trigger the transformation.
Another possibility is that she gets injured. While she is recovering, she is less active and eats a normal amount of food. But with the lack of exercise, the calories start to add up and she gains weight. Also, if she is given some sort of special medicine as part of her treatment that has a side effect of increasing appetite, that would contribute too.
She could have had a traumatic past. Maybe she witnessed a horrible event that made her shut down her emotions to protect herself.