Measured 3 days behind at ultrasound??

I thought I was 7 weeks pregnant on the dot today when I went for my ultrasound. However, during the vagina ultrasound she said the baby is measuring 6 weeks and 4 days. Heartbeat of 125. Is this normal or should I be worried? I’m questioning it because I did opks and the 7 weeks went with my positive opks. So I don’t know why I’d be measuring 3 days behind 😩