Yes it would be possible she changed in your room, because she is that type of self entitled girl.It makes sense. I think your husband could be stressing because he really doesn't like your house guest to begin with. And you have this sense of her being a trouble maker because she is being an attention whore. Well she could just be that, an attention whore and not someone who would induldge in adultry.
You should have a talk to HER about being in your room without your permission, and it is off limits to her. And see where that goes.