That happended to me at 6weeks. The dr said I was 5 weeks. I explained to the dr I know exactly when my period started and the exact day I had sex. He told me either my dates are wrong or I would be having a miscarriage. I knew my dates weren't wrong. Turns out I was right and now im 31 weeks pregnant. The dr just gives an educated guess about the situation.
I had a miscarriage last year where the embryo quit developing around 5 weeks and we didn't realize it until I had an ultrasound at 9 weeks. I had to wait a week to have another one to see if the embryo developed more. Unfortunately for me it didn't. Hopefully your baby is okay.
Sorry I have t been through that experience, though I have had a MC. Good luck to you hunni. Xxx