Here, I've created a nerd girl, I'll call her Bea, with her tall, massively muscular (were)bear form next to her.

Short Story
Teenage Bea was what people generally call a nerd. She loved reading books (especially fantasy, science fiction, horror, and history) and was good at computers. However, she was generally ridiculed because she wasn't one of the "cool" kids. Although she was very good at the most important school subjects, she wasn't appreciated by her peers; the "cute" boys didn't pay much attention to her. But Bea had no idea that everything would turn out very differently.
One day, she was walking along the school corridor when Brenda, the "hottest" girl, started making fun of her. Bea felt a great deal of anger rising within her; even more, she felt like she was literally swelling up. She felt stiff, bristling hair growing all over her body. Her face changed; her mouth and nose became bear-shaped, and her ears became bear-shaped. Bear paws burst from her sneakers, and her fingers became more paw-like. Her muscles thickened, and her clothes began to tear. She began to grow to more than two meters tall.
The transformation was complete; Bea had now become a large, massively muscular, anthropomorphic brown bear—a werebear, to be exact. Just like a werewolf, but in bear form.
Her classmates were astonished, and Brenda turned pale with horror; she couldn't even run away in terror.
Bea couldn't believe what had happened to her. At first, she was astonished by her new height; even the tallest boys in school were dwarfs compared to her. She could now see without glasses. But what fascinated her most was the enormous muscle mass. She flexed her arm and saw how thick her biceps were. She couldn't explain how or why she had transformed. Her grandmother came from Scandinavia, so she had told her stories not only about werewolves, but also about werebears, even about great warriors who could transform into bears. Were her ancestors really werebears?
At first, Bea was afraid that everyone would be afraid of her, but contrary to expectations, her classmates found it simply exciting. Brenda was stunned and called Bea a "bear freak" and asked how anyone could find such a thing exciting.
But Bea bent down until she could look Brenda directly in the eyes and let out a growl.
Brenda ran away, filled with fear.
Bea's friends cheered, as did all her other classmates. To her delight, Bea discovered that she could transform at any time.
Her parents were very surprised and couldn't explain it. Bea's grandmother was delighted that the stories she was able to tell were truly more than just legends. Naturally, her parents urged her to use her new ability responsibly and only do good things with it.
In her bear form, Bea loved striking bodybuilder poses and demonstrating her bear strength; she could even lift heavy weights. Many students, both boys and girls, were eager to take selfies with her, for which some were even willing to pay.
Naturally, Bea used her bear strength to help others, both in and out of school. She even acted as a protector if a bully dared to bully someone.
While this made Bea popular, she remained down-to-earth and stuck by her true friends, who liked her both as a human and as a werebear.
Even the most popular boy in school, his name was Mitchell, began to develop an interest in Bea. At first, Bea was happy about this, but she quickly realized that he only liked the bear, not her (she was probably more of a big teddy bear to him). But Bea found her true love in another boy, named Clive, who loved her for her personality and not just for her bear-like form.