Some guy got her drunk and broke her in college. If I recall correctly (not gonna go back and find it) she had a sports scholarship. That guy turned her into what she is today and ruined the future that she had to that point. He probably got tired of her once he'd changed so he dumped her to move on to another target.
Or maybe I just read too much of the wrong type of thing.
I'd say you're thinking in the right direction, but personally I don't think you're going dark enough.
My thoughts are that the first guy who got her drunk and 'had sex' (see also; raped) with her spread it around that she was a lightweight drunk and an easy lay, so the same thing likely happened to her on multiple occasions, to the point that she (unfortunately) got kinda used to being used as a sex toy. Just look at her reaction when they went to karaoke together. She was fully expecting that he had brought her there to have sex, because that's a fairly common use for karaoke booths (at least in hentai) for young people. There were likely some incident(s) that finally broke her spirit and made her drop out of university to come back home, and she's been basically a hikkikomori ever since.
That might also be why she changed her looks and maintains that change, because if you remember, she said that she changed how she looks. She likely doesn't want to see how she "should" look because it would bring up traumatic memories. It's a pretty common coping mechanism.