It's a dream caused by worry. It means somebody is chasing your girlfriend.
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