You haven't talked to your boyfriend in three weeks? No to be rude, but are you sure he is still your boyfriend? Unless he is in the military or some other job where he doesn't have access to some sort of communication with you, that is a long time to be out of touch.
Your dream could be realizing you aren't together, it depends on how your last conversation ended or maybe you're stressing over not hearing from him, and the dream is your fear that he is done with you. Don't stress cutie pie ^^ It could just be what you fear has happened... Dreams are composed of our subconscious fears and wants.