It sounds like she has a vivid imagination, much like myself and my son. A seemingly innocent thing seen in the wrong context could have triggered these nightmares. Maybe something that was on the news when she walked in the room, a picture in a newspaper or magazine while walking through the store etc.
I will never forget when I was about 9 or so and we went on vacation, we walked through a book store and there was a paperback the front cover was pretty innocent looking. There was a picture of a women being framed by a circular cut out, so you knew there was a bigger picture inside, when you opened the cover the women was actually being bitten by a vampire. This picture gave me nightmares for weeks! I don't think my mom even knew either.
One suggestion that worked when I babysat, was to get a spray bottle, fill it with water and label it "bad dream spray" or "monster spray" then spray that around the room before bed. I also used to go on monster patrol check closets, under beds etc before bed.
My son also has a large teddy bear in his room which we call the nightmare bear, because my son believes that he keeps nightmares away. Maybe some of these ideas may help.
I'm also a very light sleeper and wake at every sound! Hope you find something that works!