I feel like I've lost her?

I have been best friends with A for three years and we've been through a huge amount together. But recently she has been hanging out with B. I helped B out a lot last year, she had problems I later found out to be made-up. She was nice to me when she needed my help but not when she was ok. Recently a lot of her friends got fed up with her. She used to swear at them a lot and she does things like leave anonymous tweets that are offensive but people worked out it was her. She is a huge drama queen and likes to be the centre of attention. This is me being as objective as I can be after the way she used to treat me. Now, however she has no friends but is suddenly spending alll her time with A. Today was my birthday and A didn't even say "hello" to me all day. Yet on Saturday we spent the whole day together. She just hangs out with B at school and I know I have done nothing to upset her. I don't get it. I understand it is up to her who she hans out with but I can't lose her I have a lot of problems with my family right now, so does A we used to talk about this stuff but no I have other friends but nobody I can really talk to. What should I do?