Not as far as expected?

Je

So I had my first ultrasound this morning I thought I was 8 weeks turns out I’m only 7 weeks and 3 days. I just don’t see how that’s possible. I track my periods and in August I tracked my ovulation. So I thought I knew exactly the day. I knew with my son exactly. What could be the reason for this? I’m just so baffled I’m scared that maybe something is wrong. The heart rate was 163. I also found out I have a sort of hematoma at the lower part of my cervix, it’s not an actual hematoma but that’s how he described it. He said that it’s very possible I’ll have some spotting to even light bleeding but it’s probably not a reason to worry that if I do start bleeding to come back and he’ll do an ultrasound to check. Has this ever happened to anyone else?