I need your opinion about something in my relationship... PLEASE READ TO KNOW!!!!! Can you help?

There is a new guy at my girlfriend's work. My girlfriend and him started talking and stuff and added each other on facebook. One day, she told me that "she doesn't know if she's flirting with him." Next few days she IMs him saying that she broke up with me when we never did. She said that we "broke up" because of some stupid little thing. I discovered this on her IMs. I got mad and stuff. She told me that she likes him. After telling the guy the truth, my girlfriend gets mad. Now the guy acts sad and doesn't look at her; for some reason I think he's trying to get attention. He left comments on my girlfriend's facebook and continues to poke her even after "he doesn't care anymore" or he's mad. I have a feeling that the guy wants to be with my girlfriend and I want to try to find a way to stop it. She's working with him this Saturday almost a full shift. I want to know what's going on when I'm not around. Never in her life she was mad at me about something like this. HELP!