Baby is measuring small.

Im suppose to be 10 weeks 2 days today.

I was taking opk and got a positive one September 20th, 2018

I got a positive pregnancy test September 28th very faint

I had an ultrasound on Tuesday at 10 weeks and she sais I was only measuring 8 weeks 4 days.

Is it possible to actually only be 8 weeks 4 days if I ovulated September 20-22nd?