Borderline Personality Disorder
I don't know if this was brought up yet, but I'm feeling this is what Rebecca had instead of being Bipolar.
She showcases a lot of the symptoms like copying Sara's appearance.
Also was this movie supposed to be a modern version of a Single White Female? Very similar storyline except this was set in a college.
Btw I'm not trying to be ignorant or anything because Hollywood exaggerates everything. My guess is she had this and could of been a sociopath at the same time...
My momma always said there'd be days like this, days like this,days like sh!t