Ugh! Mad at doctor

4 weeks ago I had my checkup, and she said she would schedule an ultrasound for 20 weeks to find out the gender and everything. Well that appointment is supposed to be tomorrow! My husband and I have been counting down the day. Today she calls and says that something changed, and I can still have a checkup tomorrow but i cant get the aultrasound until the 19th! I'll be 22 weeks then.. I'm so upset!