Measuring behind ?


Is anyone measuring behind ? I’m measuring 10 days behind which is making me nervous. I should be like 9 weeks today according to my app and lmp. My last appointment was last week Friday and I was measuring at 6 weeks 1 day when I should have been 8 + weeks. All I could see was a gestational sac and a small bean. Should I be worried? I ovulate on time too .. not late. Anyone experience this and was ok? The doctor thought she saw a little flicker for the heartbeat