To see your girlfriend in your dream represents your waking relationship with her and how you feel about her.
To dream that you are pretending to be someone’s girlfriend implies that you are not ready to settle down into a committed relationship. You don’t want to be tied down.
Dreaming that you are watching your girlfriend have sex with someone else suggests that you are taking a passive role in your sex life. Maybe you feel that you are not satisfying her. The dream is signaling you to communicate your sexual concerns and/or desires. Alternatively, the dream may reflect a personal fantasy you have.