Why wouldn't she invite me to her birthday party?

I think that is really odd behavior for someone who is interested in you. =-/ If she likes you, the only reason I can think of for her to do that is because she wanted you to ask about the party or something, but that is still putting you in a really awkward position. As SappherAngel462 said, it's possible that one of her friends likes you, so she is backing off because she wants to protect that friendship. She may also be trying to keep you as a friend only and make sure you know that it's nothing more, though she is doing it in a roundabout and immature fashion. Either way, it's pretty unfair to you and I think you ought to look for someone better who won't play these games that cause hurt feelings.

Hmmm.... good?.. my guess would be that one of her friends likes you so she is backing off. but sorry to say but she could have really liked you as a friend and was really open to you but when she started to get the sings that you like her she got scared because she doesn't feel the same but doesn't want to lose you as a friend.

But you should tell her how you feel even if the feeling is not the same at least you tried and don't have to live with what ifs. good luck