Off by 6 days…is this normal

Hello… My first day of my period was November 18. I track my period and ovulation very closely. I’m am normal with a 28 day cycle. From the ovulation chart my fertile days were 11/26-12/1. I took an ovulation test on November 27 and it was a solid circle. I took another one on the 28th and it was a solid smiley face but it never flashed. It stayed that way til the 12/1. I took another ovulation test 12/2 and it was back to a solid circle. I had sex Nov 25th n the 28th and I did not have unprotected sex again until December 2,3 n 8th. On January 2nd I went to the ER for cramping and some spotting. I should’ve been 6 weeks 3 or 4 days with a due date of 8/24 based on lmp. The US tech did an abdominal and a transvaginal on me. I was surprised that she said I’m measuring only 5 weeks 5 days but the baby has a heartbeat of 123 bpm n my hcg was 13,381 and she gave me a due date of 8/29. Definitely was shocked I saw a heartbeat this early. I’m confused as to how I was 6 days off. I know the 8th was too close to my period to conceive on that day but is it possible? Could I still be fertile 6 days before my period? My period was supposed to start 12/14. Can anyone help me with this? I’m just worried bc they say I’m measuring behind since I’ve had 2 MC’s last year.