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.
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.
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.
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, 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.
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.
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 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.
One possible reason could be the loss of a loved one, like in the case where the cat's owner died. Another might be separation from friends or family, as the cat that lost its mouse friend. Or neglect, such as when the family focused on the new baby instead of the cat.
One possible plot could be that a magical spell is cast on the boy. Maybe he stumbles upon an old wizard's lair and accidentally triggers a spell that changes his gender. He then has to navigate a world that he previously knew only from a boy's perspective, like dealing with different social expectations in school or at home.
At first, she could be in disbelief. She might think it's just a strange dream or hallucination. But as the transformation progresses and she starts to experience the world as a cat, she may feel a mix of curiosity and fear. She'll be curious about her new form and abilities, but also scared of the unknown and how she'll survive in this new state.