That's the point. She IS a nerd. She is also Magnar's girlfriend.
She is having trouble with anxiety, depressive thoughts and etc and she is taking more and more meds than usual from Quinn and she even used Karen to get better prices.
She is having trouble with her studies which makes her even more annoyed, she is at EVERY party with or without the DIKs, like every single one. Even the one at the campfire one, which don't make sense. Magnar is basically forcing her to scout the parties so he can do his stuff, which will make her grades worse by default and will make her even more annoyed.
To top it off, on a lot of interactions we have with Sally, she ends it with a guilty/troubled face. I don't have the screenshots right now, but she does... at the dorm party, at the campfire. And she even fells guilty for threating the MC bad when he helps her with Nora's class. The guilty from whatever she has done (I even suspect she was the one that posted Cathy's cluck) is eating her on the inside. There's a lot of stuff pillying up, I will probably get screeshots of everything in a next playthrough. But she is close to snapping.