What does it mean when i dream about a girl that is not my girlfriend?

I would think that is normal.

Everyone, (at some time or other) dreams of the opposite sex... even if the person appearing in the dream is not a spouse, mate, or otherwise connected to you.

However, if you are looking to discover a meaning behind the dream, perhaps you might consider re-evaluating the relationship with your girlfriend? Other than that, a dream like that is relatively normal.