I think my boyfriend is cheating on me-for a girl! What to do?

(yes, I'm gay) I love my BF so much, but recently, whenever I'm with him doing something affectionate with him (if you can understand all that PDA stuff, the exceptions for school stuff) sometimes there is this girl who comes by. One time, my BF and me were making out and she literally came by then ran off in tears. I know I didn't do anything, but my BFs acting a bit weird to. I've been with him for sooo many years before this girl came up. Does he actually like her more than me? She just came in about two months ago. What can I do??