Why is my friend doing this?!?!?! Please help!?

So I've been best friends with this girl who we'll call Sophie for about a year and she's really the only person I trust. I've told her all of my secrets and she's told me hers. But things have changed. She started hanging out with this other girl who we'll call Megan. Sophie and I only have 2 cles together and we sit by each other in 1 of them but we sit there so she can talk to the guy she likes so her and I don't really talk very much. And in the other cl Sophie decided to move away from me and sit in the back kinda by the guy she likes. She's half way between me and him. She says the reason she moved away is cuz she didn't want to sit next to this girl that sits next to me now. It was Sophie's idea to sit where I'm sitting and now she moved somewhere else. We used to sit in the back alone so we could talk and p notes. Now we only talk a little bit in our 1st cl and we don't talk at all in our 2nd cl. Everytime her and I have the chance to talk (break, recess, lunch, after homeroom in the hall) she's talking to Megan. It's kinda been getting on my nerves especially now that she's been telling Megan REALLY bad secrets that she won't tell me. I asked if her and I were still best friends and she said she didn't know. She said that we've both made new friends (which I haven't!) and haven't been talking very much. I was crying when she said that. Her and I talked about it for about 2 days and now I'm confused. I asked her if she wanted to make our friendship work. I called her tonight about school and her and I ended up talking for about an hour and it was really nice. That's the most we've actually talked in a while. She says that I should come sit by her at lunch and stuff but if I do that I'm afriad to make her other friend(s) mad cuz they don't like me. She wants me to move to the back of the room w/ her but we wouldn't be sitting at the same table but we'd be closer. She says that she doesn't talk to me very much cuz people are always around me but trust me they're not. There's just this one girl that like stalks me and that's pretty much it. I'm so confused. Our friendship used to be so good but now it just doesn't seem like she wants to be my friend. I mean she's said stuff about me behind my back and told some bad secrets about me but I forgave her and we moved on. I sat by her at lunch like she said I should and she didn't say a single word to me. She turned her back to me and talked to Megan. I was SO upset when she said she didn't know if we were still best friends. I cried a lot and I called my big sister and she made me feel better but just the thought of me and Sophie not being best friends just crushes me. I've been there for her this whole year, I've never judged her, I've always listened when she needed me to, she knows she can trust me and always count on me. I just don't know what I've done. I don't know if we are or not cuz of what she said when we were emailing but when we talked on the phone it was like nothing had changed and our friendship was perfect. Why is she acting like this? She won't tell me secrets, she ignores me when we were at our fieldtrip on Tuesday, and she acts like this is my fault. She says I'm the one not talking to her. What do I do? Should I try having her over this week end? Should I try talking to her more at school?