My dates are way off! Nervous! Kind of long sorry..

Analicia • Baby L ❤ 1/8/15

Ok so my LMP was 10-19 which would put me around 10 weeks right now. I got my first positive pregnancy test on 11-27

Had my third ultrasound today and the baby measured 6 weeks 1 day on 12-30. Doctors act like it's fine, but it makes no sense to me! How could I ovulate a month late? My periods have always been regular.. I'm just afraid I'm going to lose this baby bcus of how small it is ?

1st ultrasound on 12-10 showed just a gestational sac, 2nd ultrasound on 12-17 showed gestational sac and yolk sac, 3rd ultrasound on 12-30 showed both plus the baby and a flicker of a heart beat. Doctor said everything is progressing like it should, but I guess I'm just scared? Thoughts/Opinions anyone? Thanks!