Change in due date

Based on my cycle and when my husband and I thought we concieved calculates to 8 weeks and 1 day (due 4/6/17) we had an ultrasound and our baby measures 7 weeks 2 days with a due day of 4/12/17. I am trying to not over think this and worry without reason. Has this happened to anyone else who regularly tracked their cycle? I counted back and that means I found out I was pregnant at 2wks and 2 days??