It means that it doesn't mean anything. Those are YOUR thoughts. Not hers.
Your dreams are about your thoughts, your feelings, your actions, and your subconscious. So....Are you feeling guilty about something? Have you done something that would make your want to break up with you? Or maybe you are feeling insecure for some reason? You have to look t yourself for the reason for the dream.
The only thing to fear is,
Its only a dream. It might not even happen.
You're probably just afraid of it. You're dreaming about it because you have fear for losing your girlfriend. Don't worry. It's not something you should worry about.