Measuring 5 days behind, anybody else?

Hi, my first ultrasound I thought I was 7w 2d but as it turned out I was measuring at 6w 6d, so I was 3 days behind what I thought. Heartbeat was 121. a few days later I had another ultrasound due to some strange pain and worry... I was then measuring to be 7w 2d when originally I would have been 8 weeks so at that point I was measuring 5 days behind my original due date.... is this normal or should I worry? Heartbeat on second ultrasound was 141. I know it’s not good for me to worry but it’s all I’ve been able to do.