Measuring 10 days behind at ultrasound

Kathleen • MMC 9.24.17 MMC 01.12.18 Rainbow Baby Girl 03.08.19

I had a chemical back in June so when I got my BFP in August my doctor started testing my HCG (slow to rise and then started doubling normally but doctor stopped testing once I reached 633) and then wanted me to come in for an ultrasound. Based on charting I thought I was 6 weeks and 5 days but according to the ultrasound I'm 5 weeks 2 days. Is this normal to measure this far behind? They only saw a gestational sac so I'm pretty nervous 😩