Re-occurring dream about a girl I don't know.

Last night in my dream, I was generally hanging out and walking around with diffrent people and going onto diffrent floors of our school building, but there was this beautiful girl who was my (in the dream) ex girlfriend. No one told me, I just kinda knew. She just kinda talked to me a while, and showed interest in me, and I kept my friend away from her when he tried to talk to her.

The problem here is, I haven't had a girlfriend in over a year and I wouldn't ever want to get back with her anyways because she slept with somebody else.

So who is this girl that I keep seeing and why is she so, perfect? It doesn't make any sense to me.