ultrasound showing I'm earlier than thought

Faith
So with having a positive opk and tracking my cycle super well I put myself at 5 weeks and 6 days when I had an ultrasound done earlier this week. I'm concerned because they measured baby at 5 weeks and 1 day?? Maybe I'm just freaking out because I'm still early but it doesn't seem right to me. 
Also I had an ultrasound done so early because I had a miscarriage before getting pregnant again this time so this is my rainbow baby. 
Anyway... anyone ever had the radiologist measure wrong? Or measure the baby earlier and everything be okay?