My girlfriend dreamed that I had an argument with someone and was shot.

This is a typical love possessiveness, a dream of "trying to completely possess the other person by letting the person you like die in the dream". Usually this type of dream will dream that you (or a third party factor) killed the other person, but whether the other person is alive (I don't want the other person to really die) and so on, in order to achieve a sense of control from the psychological balance. Usually the reason for this dream is out of complete love and insecurity. You should spend more time patiently communicating with her.