I’m 14 weeks and 3 days pregnant. I moved March 18th and got with my now ex that I knew over 2 years. We started having sex in the 21st of March and I got my period March 26 - April 1st according from that period my due date was supposed to be December 31st when would I have gotten pregnant?

My dr measured the baby and pushed me back a week bc the measurements were measuring smaller which I heard an ultrasound is more accurate so my new due date is January 8th 2021 when would I have gotten pregnant?