I’m supposed to be 7 weeks..

I had my first ultrasound yesterday and I was supposed to be 7 weeks based on my last missed period which is September 4th. I got my ultrasound done and I’m apparently measuring less than 7 weeks, I know that I ovulated later then I was supposed to, I’m not sure on the exact date because I stopped testing 2 day’s after my said “ovulation date” because I was getting negatives. I got a CLEAR positive pregnancy test on October 6th. But my ultrasound looks around 4 weeks, he did say he could see a tiny spec that would be the embryo, I’m going back in 2 weeks for another ultrasound. Anyone else had this issue? He said he wasn’t concerned and it’s not uncommon but I’m still worried