You can always say you have a lot of schoolwork to do! (Something to justify staying in your room....)

I never liked roommates to bring in too many guests.

I am more introverted. There's nothing "wrong" with that. I was never talked about for being that way.(As far as I know. I was seen as more shy, I think.) Of course, we don't know for sure that she was talking about you in a negative way. Or did you actually hear word for word what she said and you know it was about you?