I use to have the same dream, I always dreamt my Fiance was cheating on me or meeting a new girl or having sex with a friend of mine. It is an ugly feeling I was always dreaming these type of dreams because I had major trust issues with him. You need to talk to him about this and figure out if you can learn to trust him or move on. We all need to have trust if we want our relationships to work.
Your brain needs time off...stop thinking about it ( if there is no reason ) and let it go...you are driving yourself crazy...it is just a dream....
It's either your just feeling insecure OR it is your intuition telling you that you are right! If unsure, then maybe check his cell phone from time to time. Have you asked your bestfriend anything? You don't have to be rude when you ask her though, just in case there is nothing going on.
You can also try going out alone with her one weekend and talk. Then if you go to a movie or a club, call your boyfriend to meet up. When he shows up leave them together and see if they act differently. You have to stay distant but close enough to see anything. I'm only suggesting this if you are really concerned. Don't ever follow him because it could be grounds for stalking.