Something Isn’t Adding Up.?.

I went in for my first ultrasound (should be 10 weeks) but the fetus was about the size of 6 weeks. They believe the saw a heartbeat but obviously something is off since based on my last period I should be 10 weeks and I had a positive pregnancy test 6 weeks ago which means if I am 6 weeks pregnant I would have had a positive days after getting pregnant. Has anyone else experienced this before?

I have a follow up ultrasound in 2 weeks but I’m just really confused about what’s happening.