Is this girl really my friend or not????????

No she`s not a true friend, real friends don`t pretend to be a friend one minute and the next become really cruel and nasty. I understand WHY she`s doing this, she wants approval from you, and from these other girls too, basically she wants to be your friend, but be there friend too which means picking on you. You need to tell her that she`s not a REAL friend, and that what she did has hurt you. Then walk away from her, 2 things will happen either a) she will come back to you, apologize for what she did to you or b) she will stay with these other girls and be their friend.