W.A.
I had the same problem and ended up talking to my pediatrician about it. She told me that this is a testing time. She is testing you to see if you will come to her attention when she wakes up. The best thing to do (and keep in mind, it's going to be hard) is to just ignore her. For a couple nights just try letting her cry, scream, fuss, etc until she falls back asleep. Once she realizes that you aren't going to come to her, she will stop waking at night and get back on a regular schedule. (this took my son about 4 nights)
Good luck!