Late ovulation?

Geri

So I had a scan the other day. I was supposed to be 5w4d but doc saw a very small sac and said that I’m maybe 3-4w. I know my ovulation was a few days later than normal because I used an opk and if I count from the day I ovulated to my scan it was exactly 3w. So how can I be 2 weeks behind. Is this normal? Is anyone’s scan also taking them back that much. I’m worried if there will be a baby in that sac.