2 weeks off on how far along

I had an ultrasound at 5 weeks (by my last period date) the doctor said the baby was measuring 3 weeks. I have a new doctor & my second ultrasound on Thursday, where I will either be 10 weeks or 8 weeks. Anyone ever have the baby measure less than expected & then going to an ultrasound a bit further down the line, the baby was measuring back on track to what you thought originally? Or did your due date change?