Friend vs. boyfriend...HELP!?

Well, I am seriously mad right now. hah. Just thought I'd say that...My *best* friend CONSTANTLY talks to my boyfriend. Whenever I am talking to him she s into the conversation and totally changes the subject and basically cutts me out of it. And this hasn't happened just a few times, it happens all the time. I think that she likes him but she would never admit it because i am going out with him. But it just makes me so mad because I don't normally see him a whole lot because he's a grade older than me, so I really only see him in homeroom and chorus! I don't really want to say anything to her because she has a way of telling everything i say to her to my boyfriend. What the heck do I do?