Anyone have similar experience??

According to my calculations and apps I should be about 5 weeks and 4 days along. However, I went in to Ultrasona yesterday to get an early ultrasound done since my OB won't see me until 8 weeks. The ultrasound tech said my gestational sac is measuring at 4 weeks and 4 days. There is a tiny yolk there which is a good sign but I'm really worried. I'm not sure why it would be measuring a full week behind. I got a positive pregnancy test two weeks ago so that would have put me at two weeks along and getting a positive test, which doesn't seem possible. I'm not sure if their test is off or if the baby has stopped growing and I am going to miscarry. Has anyone had a similar experience? I'm so worried!!