From everything I know and read, early ultrasounds are fairly accurate within a few days either way. Are they scheduling you another ultrasound? There a few reasons why their dates wouldn't match yours. It is possible that you ovulated later than you thought and implantation can take 1-2 weeks. So, for instance, if you ovulated a week late and then it took 2 weeks for implantation, there would be your three week difference between your date and theirs. Try and stay positive and wait it out. I know, easier said than done. :)
I was 9.5 weeks when I had my first ultrasound with my daughter, but the ultrasound tech said I was only measuring 7.5 weeks.
Ultrasounds can be off by a week or two in early pregnancy.