Ultrasound!!!!! Advice please :'(

I had my first ultrasound last week at the estimate of 6 weeks 6 days. They technition said everything looked fine to come back in a couple of weeks as it was too early to tell how many weeks. Went to the doctors today and she said that the technition said my sac was measuring in the 5 week period last week(which he never told me) she seemed worried about and not so positive. When I had my ultrasound I saw the heartbeat flickering on the screen which wasn't related back to my doctor.