Not as far along as I thought

Ali
Okay, I'm confused! And so was my doctor! 
According to my last period, and the day I ovulated, which was confirmed by an OPK, I am 8w6d. 
I went for the first appt today and the ultrasound tech put me at 7w3d, and then, gave me a later due date. 
When I met with my doctor she was like, that's not right, but then chalked it up that I must have ovulated 10 days later. 
I'm so confused!!! I'm ALWAYS regular and I took an Ovulation Predictor kit that gave me a 100% positive for July 1st ovulation. 
To be honest I looked at the date the ultrasound tech said we should have conceived and my husband had a crazy work schedule that week, so I don't even think we had any sex. 
Has this ever happened to anyone else?