Boyfriend's bestfriend.

there's this guy, Joe, and I have been with him for about four months and things have been going great.. im crazy about him. But the only problem is his bestfriend is a girl, and she picks fights with him all the time saying things like he has changed...

since he has been with me and even though he is still her bestfriend, she doesnt know him anymore. They fight about that, and they also fight about me. The weird thing is she is the only one who notices him acting different. At first I thought she might like him.. but she is with Joe's other bestfriend.. so that idea flew out the window fast. The thing that has always gotten me is she has always been real nice to me and we hung out without him there too. So I dont see how when he called I told him I had to go and she calls me back and yells at me, telling me to call joe back and stuff, but when I do and asked why she called me she tells me to shut up she never liked me and all this stuff.
it just seems to much because of one guy, you know?
any advice or any idea what is going on in this girls head.

She sounds like she is making up things to break up guys up. Or to see who he likes better you or her.