Help! Measuring behind in early pregnancy!

Bailey

Recently I went for my first ultrasound. I was expected to be 8 weeks and 1 day. However, my dr said I’m actually only 4-5 weeks. He for sure sees one sac but said he is pretty sure he sees 2. We are due for another ultrasound on Thursday to hopefully see everything better. Should I be concerned I’m so far off? I know for a fact when I started my lmp. However, I got positive opk on cycle day 17. First positive pregnancy digital test was 11dpo.