Ultrasound dates

I had an ultrasound at 7 weeks but measured 6+5. That dating was accurate because I ovulated CD16. I went today for a repeat ultrasound at 8+3 and baby only measured 7+3 but had a heart rate of 166. Before I measured 2 days behind now it’s a week. Is this really bad? I’m so upset that it went from 2 to 7 days behind. The doctor said the heartbeat is strong but I’m scared the baby isn’t growing as fast as it should? Did this happen to anyone and it was okay? What should I expect?