Is my Best Friend fake?

My best friend since kindergarten, cathy, is getting on my nerves. Before middle school, we did everything together, even though, we had more than 1 bff. Since middle school, it seems like shes become fake. She found 3 other bestfriends. she does everything with them and not me. they all go places together and don't invite me! I think this is wrong. she even invited all her bffs over for a sleep over and they came up with their own little name for their "group" its call WESAAF- wont ever stop an amigas friendship.. and when shes with me, shes herself, i would know, ive been her bff since kindergarten, but when we are all together, she a totally different person! i dont know what to do...