Dating Ultrasound Help?

I’m very worried because I think my baby measured small and the due date is wrong.

This is the situation - on October 26th I took an ovulation test that was positive. On November 15th I went in for surgery only to find out I was pregnant and had a 406 HCG level (didn’t have surgery). On my first ultrasound on December 18th the baby measured exactly 7 weeks, a week behind what I thought.

According to my math on dates, this doesn’t seem right? According to the new dates, when my HCG was tested I was only two weeks? I know dating scans are supposed to be accurate, but I’m worried by baby measured small because they are actually small... the heartbeat was also not very high at 125 bpm.