It was rude of her to invite you (regardless of how it came up) and then take it back. That being said, you're handling it nicely. And trust me, there will be plenty more weddings and friends asking you to be a bridesmaid!
So enjoy the wedding, get people to sign the guestbook and relax-- have fun and be excited about the fact that you can wear whatever you want, wear comfy shoes and you don't have to shell out a ton of money for a bridal shower!