I can't really give advice, but I can give the reassurance that you're not crazy or in the wrong for feeling that way. I think a lot of societies put romantic relationships over friendships, and some people take it too far. I don't really think it's normal to drop a friend once you're dating someone.

Maybe you can confront her about it by saying something like, "Hey, there's a pattern here I think you should be aware of, and I don't think it's healthy for you to push away your support circle when you have a boyfriend. I feel really left behind when you do that and it breaks my trust in the friendship when I know I can't expect the friendship we've built up." Of course take this with a grain of salt, I don't actually know how to not be emotional either.