Why is my 'bestie' acting like this? shes changed since we started highschool!?

[XxVerXx]

My bestfriend and i were friends in primary school and became especially close last year. We always told eachother everything and i would be the first one she would always invite to things. At the beginning of high school every thing was normal, but then after the third week everything suddenly changed. She was always trying to get away from me, she started calling me embarrasing and annoying. I told a mutual friend and she asked her why she was being like that but she denied that she was, and then she was being so negative all the time. She use to be the one who always cheered us up and now shes always so down. I tried talking to her on Facebook but she said she cant be bothered. And shes always caring about popularity. She never talks to me anymore. When she does its so formal. Its only the fifth week of highschool and shes successfully distancing herself from me. Also, there was someone we both hated, but now she says that person is actually pretty cool. Why is she being like this?