Confused about dates

Ka

The first day of my last period was September 12th. (Very regular periods) I have driven myself crazy as I've been ttc for 3 years so my rule has been 1 test a month.

I missed my 1st period October 10th. Negative test. Had to wait it out. I missed my 2nd period November 8th. Took a test. Big, fast and dark positive! At this point I assumed I was pregnant all along! Went in for my first ultrasound. They couldn't see the baby on the abdominal ultrasound. And couldn't find a heartbeat. So we looked transvaginally and she said "oh that explains it! You're only about 5 weeks!" The doughnut shape in the ultrasound is the yolk. And the little peanut shape underneath is the baby. But how does that make sense? We scheduled another ultrasound for next week and did a blood panel to get hcg levels confirmed. Anyone else have an experience like this?