What does he mean his girl?

So I met a guy yesterday, and we had a short conversation. He told me that he was new to the university and that he didn't really know anyone, so he asked for my number so that we could hang out sometime. Today at the gym I saw him and one of my friends working out together, My friend, J, was flirting with me alittle, and all of a sudden the guy says, "dude stop messin with my girl". WHAT?! I just ,met this guy. What do u think this means?