Why does my friend hate me?

Ok this is the story. My friend lets call her "M", went to her dad's house for the summer. 8 hrs away! Anyway she has a group of friends there let's call them "B" and "A". Anyway B told M she couldn't be my friend. M and I got into a huge fight about it and we were not friends. I apologized then we were friends again. This morning M told me B and A said they hated me and I couldn't be her friend. I told her "You hang out with B and A only for 3 months! And me 9 months! Please don't do this again!" M replied, "Yeah but I've know them since I was two and you since last year." I broke down crying that night. And don't understand. B and A don't even know me. Why would they brainwash my BEST friend to hate me? Can't we all just be friends? But no... Why?