No yolk sack 6weeks

I have been having sharp pains without bleeding around my right ovary. My ob sent me for an emergency ultrasound to rule out an ectopic. There was a gestational sac but not yolk sack in my uterus. I only measured 5w0d so based on that gestational age there should not be a yoke sack. I  very confused my LMP was 7/27 and I had a + opk on 8/13. So based on the LMP, I should have been 6w1d and based on the opk 5w4d. Based on the 5w0d measurement, I should have ovulated 5 days after my + opk and gotten a positive pregnancy test 6 dpo. Should I be worried about a blighted ovum?