I would say that something has happened ot how would she know or be saying these things, check it out before you disbelieve your child, why would you think it is an accident ?
I had a similar situation with my nephew. After a lot of dicsussion with him, we learned that what he spoke about was something that was actually occurring with a child friend of my nephew. He was not exactly describing events in such a way that we were completely convinced they were really happening to him, but we still had to address the situation with him. Fortunately through a lot of open conversation we were able to get to the root of the concern. I'd suggest you try the same thing- speak openly and honestly with her. Be sure she knows that whatever is happening is not HER fault and she is not a bad person for telliing you how she feels. The dream may have been something she saw on t.v or heard about and is confused about what it actually means. Again, I'd encourage her to talk to you and let her know she is in a "safe place" while doing so. Good luck. :)