Dates off by over a week when sure of ovulation

I had my first ultrasound yesterday. I was 7w0d +/- 1 day by my bbt and ovulation tests. The baby was measuring between 5w5d and 6w2d with a tiny heartbeat flicker they couldn’t quite measure. They didn’t seem concerned and said my dates were probably off, but I had a positive test 10/3 at 9 dpo so they can’t be that far off 🤷‍♀️ Has anyone had an early ultrasound be off by this much and everything be okay? My last pregnancy ended in a miscarriage so I’m freaking out. I go back next week for a follow up ultrasound.