Due date adjusted

Had my first ultrasound today at what I thought was at 8 wks1day accoring to my lmp and this app however, the fetus measured 7 wks 3 days and by OB adjusted my due date by 5 days.  My cycles were ranging anywhere from 25-28 days since stopping my OCPs in October.  Given this, and a the prescence of a strong heartbeat, my OB felt confident with the size and dates and reassured me that she wasn't worried.  I am being such a worry wort and can't help but sit here and perseverate on it ☹️.  What have your experiences been with this?