Baby measuring small?

I am pregnant with my second child and from my period and the date of my ovulation (I know the dates of when I ovulated because I used an ovulation kit) I should be 8 weeks and 1 day. I took a pregnancy test 1 day after my missed period. I had some bleeding and ended up going to the ER to make sure everything was okay and the doctor said I am only 4 weeks along which does not add up. I also know that my husband and I had sex 1 time during my ovulation and not again until after I took a pregnancy test. It was a busy month so our sex life was very slow that month.

Also, that would mean I was only 2 weeks pregnant when I took a pregnancy test and it would not have picked up the hormones that early. Has anyone ever had this happen to them? I’m not sure what to do.