I am so confused. Please help!?

Okay, well, I know this girl and we have been friends for like 2 years now. Best friends, the inseparable kind. Her name is Sophie. So, anyways we do everything together. Lately though she has been a little distant and always hanging out with my other friend, Tracy. Tracy is a PAIN, always making fun of me ('why are you sitting next to me?' 'you know your really annoying and have no friends, right?') and then Tracy's mood changes COMPLETELY saying things like 'wanna come over today? we can have a sleepover!'. (I already had a fight with her about her 'comments' and she promised she stop, but only did for like a day) So anyways it seems like Sophie, MY best friend is always following Tracy around and asking me to like move over when I'm sitting next to her. What should I do?