It's a dream caused by worry. It means somebody is chasing your girlfriend.
What Does It Mean When You Dream About Your Girlfriend Being Pregnant But Your Not The Father?
It could be a premonition, but very unlikely. More likely is that it is just your sub-conscious trying to work out things. Perhaps there is something else about your girlfriend that you are wondering about. Is there are trust issue? Sometimes our dreams just provide a safe place for us to work out our feelings, i.e. If she was pregnant with someone else's child, how would you feel? So more than likely it is nothing