South Park Headcanons

Jennys Grudge

When Cartman made a deal with the school staff to prevent Pete Melman from committing suicide, he sought to accomplish that by simply replacing Pete with Jenny Simons. That way Pete Melman could feel better about his mishap and feel glad that he was not in Jennys place.
In order to make this happen, Cartman gave Jenny cupcakes with laxatives which she would eat later.
During class she would get cramps and her stomach was aching, shortly after that she got diarrhea and defecated in her pants noisily in front of the whole class.
This event shook her up so much that she became suicidal, jumped from the school building and fractured her pelvis.

When she was taken in as a patient in hospital, the laxatives she consumed were discovered very quickly. When she learned about how Cartman broke her trust, toyed with her feelings and made her give up on life, she swore that she would change.
She would change to never fall for anyone seeking to abuse her ever again.
She would change to make Cartman pay.
She soon starts developing her own sinister plans to take revenge, just like Scott Tenorman.