Can I have help on friendship problems?

My best friend and I have known each other for a long time. I thought we were true blue, but now I’m having second thoughts. Another girl has butted in, and pushed us apart it seems. My friend now seems like a different person. Now she only hangs out with the other girl. They try to wear the same clothes, they have indside jokes, they both are intrested in boys. I have no idea what has happend. And now my friend is worried about how she looks, and what clothes to wear. I feel betrayed, because now she pretends that I don’t even exsist. Before, we were perfectly happy in our own group of dorks, but now she has left me, to join the popular group. Can anyone help me on what I should do? Should I make new friends? Should I still try to win her back? And can someone please tell me why my friend is like this?