Best friend...!??? PLEASE HELP!?

I have this friend called Mia, we have been best friends for 6 months, going to each others house nearly everyday and been very close. She calls me names sometimes, like thick and stupid and it hurts me deep inside but she kinda means it like a joke. But whenever I call her something she goes into a mive mood with me. She will ignore me for days not saying anything to me. Then one day I lent my head phones to this girl who didn't have any and she got into a big mood with me because I was posed to share them with her. Now she is hanging out with these two tarts, me and my other two best friends molly and May really hate her. She completly ignores us and recently she has been threating me and sending abusive texts. But i sent her mean texts back to. Now she's getting her big sister to ring me up and abuse me. Im scared.