She either was already cheating on another dude and can't be too consistent with you or tip off her man
Or she one of those chick's that knows how to work her own nut and you really didn't hit it as good as you think.
Or she could have been banging you trying to get over another guy and only felt bad afterwards or any other personal situation that hurt the girl and she thought having sex would make her feel better
Some women are not permanent thots, some are only temporary thots where they act outside of themselves cause they hurt then once they get that out they system they go back to being a good girl then live to tell they story of when they was thot'n for awhile.