Well even if it did happen while you were sleeping, it also could've been Lavender or Parvati. They're in your house, they could come into your dorm while you were sleeping. Unless Gryffindor House keeps the girls out of the boys' dorm as well as the boys out of the girls'.

If it was at breakfast it could've been anyone, I mean Padma has loads of friends and some of them are very clever especially at hexing and curses. Draco, for instance, but I doubt it was him because he wasn't around laughing at you yesterday. I don't know what he's up to, he's been kind of distracted lately.