S.N.
My son went through some nightmares awhile back. When a person sleeps, dreams are the brain's way of processing everything that happens in the day. At the preschool age, everything is so new, that little kids have alot to process at night. This is the time to analyze what you are watching on television on a daily basis. To a small child, even the evening news could have some scary images for a child to process. Even some childrens' shows can be the culprit. I talked to my son as he was calming down after a nightmare, while the dream was still fresh in his mind. He said he had a bad dream about ghosts. It took me awhile to figure out the culprit. But it was a cartoon on Nickelodeon, called Danny Phantom. He had recently become quite fond of the show. My husband thought I was nuts, when I told him that I thought that tame cartoon was the cause of the nightmares. How could a show our son loves cause him bad dreams? Well, we tried a ban on that show, and the nightmares stopped that very same day. Even I was shocked it was that easy.
Just take a hard look at everything she might see, from her perspective. You should be able to find something that is causing it. Another idea is the nightly monster and bad guy patrol. I used to have to open all the blinds each night, check for monsters, lock the windows, lower the blinds, then tuck in my son with his special stuffed bunny(his personal security object since he was born). I told him that he was a very lucky little boy to have a bunny as his pal, because monsters are very afraid of bunnies.lol It worked.