Is she in a community tank? Sometimes aggression can happen overnight, when you can't see it. What are some of the tankmates? What size tank?
Or if it's not aggression from the other fish, is she acting differently? Is she eating? Does she have any frayed fins or white fuzzy stuff on her? (these probably require some medicine).
Sorry to answer your question with more questions, but it may help to know these things. Meanwhile, I would just keep a watchful eye on her.