What does a dream about arguing with your ex-girlfriend mean?

It isn't always clear what dreams mean and it would be hard for me to say without more info about you and the person you had the dream about. I would also need to no more about your dream. There are some psychologists that can interpret dreams, as well as a few other experts. There are some books on the subject as well. Most likely, without knowing more about your dream. It means there is something you are upset with that person about or are at odds with that person about something.