I don't think it's bad dreams. Does it happen 1-3 hours after you put her to sleep? It's probably what they refer to as "night waking" or "night terrors" (in older kids). She screams and screams but she's not really awake, and the only thing you can do is flip on the lights and suddenly she "snaps out of it"? If this is the case, it's just a disruption in her sleep cycles. Believe it or not, the best thing to do is let her scream it out. When you flip on the lights or nurse her you're just waking her up. She will learn to settle herself and fall back asleep very quickly if you just stand near the doorway and make sure she doesn't hurt herself (not really an issue with a 6 month old, though when older kids do this they sometimes sit/stand up - in that case laying them back down and tucking them in will help.
The Ferber book (Solving your child's sleep problems) actually has a really good explanation of why this occurs - I believe it's pretty common. Our son did this around that age too. The thing that caused us to be OK with letting him cry was that he was definintely not awake, and it seemed to be more disruptive for him if we intervened than if we just left him alone.