Why is she doing this to me?

You didnt do anything wrong. It seems who you thought was your "friend" was just using you. Although it's selfish and down right low it happens more often then not. Family takes advantage of one another which is why you should always pick your friends wisely. I would confront her on it. Maybe in a brief text message stating that you saw the message from FB and would just like her to know that you think that is a cowardly way of going about things and if she had a problem with you then she couldve said it to YOU. After that be done with it. She is completely self absorbed and cares about no one but herself. Trust me you'll meet alot of people like that in your life. Keep your eyes open this way you do not get taken advantage of next time. A friend will be your friend one on one or with a group of people. She, my dear, is not a friend and wouldnt know how to be one. Hope this helps.