Ignored by best friend...

My best friend cancelled our plans, saying she had to so a family thing. No big deal, right? Except my other friend saw her bowling with the girl she's becoming increasingly close with. She has been ignoring me, but i could deal. I feel like this is the final straw and that she wants nothing more to do with me. I just don't know how to deal with being left on the doorstep. I have been thinking about what i have ever done, and can't think of anything i would have done to deserve to be treated this way.