Betrayal by close friend ..help!?

i'm not sure what to do. me and my friend have been close for awhile and now shes blowing me off. nothing even happened so i'm not sure the cause. i know it sounds dumb to some people but i thought it was a little odd why she up and got rid of me as here 'sister' on facebook. i dont know what i should say to her, or if i should say anything at all. another thing i thought was odd is literally all her friends she has now, shes bashed and said she hates them to me. and now shes friends with all of them. any advice? what should i do!