Why do I dream I'm the opposite gender?

I am a female in real life but I always dream that I am a male, except in my nightmares then I'm just me. I even had a sexual dream where I was a male and was with a female, then the one dream that really scared me was I was a male and I got turned on by a female and I woke up and I was really turned on. Whats wrong with me? What does this mean?