I remembered the broad strokes, so I thumbed back through the book.
She had a kid as a teenager and married the father. They were on the outs when she came down to film, so she was in a very different place than the rest of the cast. She apparently was flirtatious with a lot of the guys (particularly Hauser) but had her eye on Jason London, who was with Chrisse Harnos (Kaye) during filming.
All of the female cast auditioned for Jodi, but got lesser parts. According to Burke, the other girls, particularly Joey Lauren Adams and Parker Posey were all very bitchy to her because she got the lead. The Shavonne/Simone/Darla car scene was written by Posey and JLA ("that bitch, I'm going to kick her ass...I'm not mad"). Per Burke, that scene was them bitching about her. For JLA and Posey's part, they don't remember being purposefully mean to her, but their roles in the movie were the mean girls and they admit they may have let it bleed into the offscreen dynamics.
Basically, she was in a weird place and may have read too much into the other girl's relationship to her. She just didn't fit in with the rest of the crowd.
Anyway, the book is a real fun read.