Measuring behind

I had my first ultrasound today and supposed to be 6w5d based off my ovulation date. I was measuring 6w1d. Anyone have similar experiences?

I am certain of my dates as I check ovulation through OPKs and confirm with BBT. I also had my first positive pregnancy test on 8DPO so I really could not have ovulated and then implanted later than I thought. Please let me know what you guys think.