In some very rare cases in real - life, there could be teratomas, which are a type of tumor that can sometimes have tissue that resembles a tail. But this is a very different situation from a normal 'growing a tail' as in a fictional story.
It could be part of a fictional story where magic or some strange experiment gone wrong is involved. For example, if it's a fantasy story, a spell might have backfired on the girl and caused the tail to grow.
One reason could be proper nutrition. Eating foods rich in calcium like dairy products, and vitamins such as fruits and vegetables helps bones grow. Another is regular exercise. Exercises like running, jumping or sports like basketball can stimulate growth hormones. Also, getting enough sleep is crucial. During sleep, the body releases growth hormones which contribute to height increase.
Genetics. If the girl has taller relatives in her family compared to the boy's family, she may inherit genes for greater height. For example, if her parents are both tall, she has a higher chance of being tall.
In a fictional sense, it could be due to some magical or supernatural event. For example, in a fantasy story, a spell gone wrong might cause such a transformation.
Perhaps it's a fictional, magical story. In a world of magic, a spell might have been cast on the girl, and as a result, she starts to grow fur. It could be part of a larger plot where she has to overcome this strange transformation and find a way to break the spell.
She could have had a traumatic past. Maybe she witnessed a horrible event that made her shut down her emotions to protect herself.
In a girl heartbroken story, perhaps she had high hopes for something that didn't come to fruition. For example, she might have dreamed of getting into a certain school or getting a particular job and then was rejected. It could also be that she was bullied at school or in her social circle. Constant mean remarks and exclusion can really shatter a girl's heart.
Well, there are several possibilities. She might have gotten lost, or someone could have intentionally taken her. Another option is that she wanted to start a new life on her own and disappeared intentionally.
It could be a part of a creative writing exercise. If she is into writing, she might be exploring different and edgy themes for her stories. For example, in some advanced creative writing courses, students are encouraged to step out of their comfort zones and write about unusual or controversial topics to develop their storytelling skills.
Perhaps it was a side effect of some strange experiment. She might have been exposed to a new type of chemical or radiation in a science lab accident that caused her cells to contract slowly.
Maybe it's a curse. Some evil magic might have been cast upon her, turning her into a cat against her will.