My best friend is being a brat? What do I do?

Okay, so I've been BEST friends with this girl since we were in first grade. (we are in 7th grade now) and all of a sudden she's pretending to be someone she's not, she's trying to be one of those popular people, and she likes this guy and she is always saying like "he was staring at me in language arts and I just don't know why" and we both know very well she is just trying to make me jealous. And there is this girl in our school, she's the most popular girl in the whole school probably and she wears a side ponytail, and my friend is copying it. She's just being a huge brat, trying to subtly make me jealous and I don't know how to go about this problem. And I told her who I like and she smirks every time I talk about him because he is not the most popular person and I'm just sick and tired of it. What do I do?