Why is my friend like this?

Lately, by BFF has been acting really cold towards me. For example, she thinks I copy her for the littlest things (If she wants to play basketball, and I say, that sounds cool, she'll be like, why do you always copy me). Also, she invites other girls over to her house and then rubs it in my face saying something like, oh, and today, so and so is coming over to swim and we'll have so much fun!!!! She also acts mad at me al the time, and when I ask her whats wrong, she's like nothing, in this real grumpy voice. And what kills me is when she does that, she goes over to talk to the other girls and laughs and jokes!!!! Its really pissing me off and I've just about had it!