What do you think of my friend?

i have a friend name May and she seems to get angry really often these past two years :/ ever since i stopped being in her cl she became an entirely different person! She became stubborn, selfish, she thinks she's a know it all, A teaser, someone who keeps mocking about someone, and before we had a friend but she became kind of mean, and i said what should we do about her she said we should try to make her a little nicer. Now she so mean to her and whenever she goes over to us she will say "What are you doing here!?! -.-" How could someone become so different? We seem to fight a lot now too. but after a fight i try to not fight and everything went fine, we were nice to each other and we had fun with each other, but On my other friends birthday, we went over to her house and me and our friend plays piano but May doesn't seem very interested in it. So since we got there May wanted to go to her room but she told her not to so she didn't but she kept saying i wanna go to your room. but we didn't, so we did piano instead, i asked her to play a song for us and while i was pointing out where she was, May said can we play wii now? :( i said "wait after she finish playing piano" then she started hitting me and i was like" hey why did you that for??" She hit me again and then i try to hit her, well i was expecting to hit her, then everyone went silent and My other friend came over and they play on the wii, May was playing on her ipod, and After i was having fun with my other friends, My other friends told her to stand up and play but she didn't and every time i say why are you being like this she says " why do you care? Just play on the piano!" i really don't know what to do about her!! how can she not care about making the mood so depressing?? My friend was sad that day and it was her birthday :( Ughh she really had become insensitive! what should i do about her? My friends said they will do something about it once they weren't mad at them, but after all she was never mad :/ now their just busy having fun keeping her company :/ they sometime leave me out :( Oh yeah she also said " what was the point of me coming here? Just to play on the piano?! I rather stay at home then come to this party!!" uhh and today my friend had a book so May said "omg you have this??!" i said can i look at it and she just kept yanking it with all her force, like she won't want me to touch it, then my other friend just try to get it for me but she just kept on holding on to it, Now that i think about it, it seems to really hurt :( Oh yeah she always ditches me and my friend for a different group of friends :/ And she always respect them...