Due date changed

I went in for my 8 week ultrasound and only measured 6w 6d so my due date was changed to June 20th. I’m a bit worried as I knew the day I ovulated, or at least thought I did...anyways she wants me to come back in on the 15th for another ultrasound to see if anything has changed and we can get better pictures of the little bean. Anyone else had their dates changed after an ultrasound? Didn’t have this happen with my first so I can’t help but compare 🤦🏼‍♀️