Measuring 6 days behind

LMP: 2/11

Positive ovulation test/LH surge: 2/27

Last day of BD: 3/1

Positive pregnancy test: 3/10

Expected day of period: 3/11

Should be 8 weeks 2 days based off LMP

First Ultrasound showed 6 days behind, so 7 weeks 3 days.

Heartbeat: 139 BPM and HCG 39,856

I’m confused how I can be measuring so far off and worrying if this may be a sign of upcoming miscarriage. Checking HCG levels again on Monday and a wellness ultrasound sound in 2 weeks. Still nursing my 2 year old a little and I know this can throw off cycles. Over-analyzing this since we had a missed miscarriage in October at 11.5 weeks.

Any thoughts or experience?