Whyd she change so much?

so ive had a friend ever since pre-k im in the second year of high school now, in about um 5th grade id say, we had the longest convo ever about how we would never change, always be there for eachother, and no matter what happens we'd have eachothers back when everyone 'goes crazy' in jr high school. so, now, she completely changed. id say it started about 7th grade. it never happened b4, but now she talks about me behind my back, and she is everything we promised eachother we wouldnt be. ive stayed the same, and i know people change in jr high but i didnt, and she promised she wouldnt either. its okay, because i do have other friends, its just i feel like i cant have a true best friend because of her, she lost my trust. and now i feel like i cant truly count on anyone but my family. is this true ? why did she backstabb me like that. she goes around and says im 'too nice' last time i checked i did not think that was a bad thing. please help. thank you:)