I'm going to assume you are teenagers and let you in on something I learned in high school; Teenage girls can be the most hurtful people in the world, for no reason other than they are trying very hard to fit in.
I have learned over the years that all of my girlfriends from high school were "frienemies", on again off again besties. No matter how close we all were, if there was a girl in the group that didn't like another girl, she managed to get everyone talking shit about the one she didn't like.
It's need for acceptance, a need to fit in and is a huge cause for depression among the teenage community. The best thing you can do is ignore it. She may be talking behind your back, but it's just because she's trying desperately to fit in. Even the popular girls in school are this way, it does get better, in collage and after. Years from now you will look back on it and laugh.