First ultrasound.... Due 12 days later than i thought.

I thought I was almost 8 weeks pregnant based on lmp. But 2 days ago I started spotting and it terrified me. I had a vaginal ultrasound yesterday to make sure everything was OK. The tech said the baby looks good and the heart beat was good, but that I was measuring at 6 weeks 1 day, not 7 weeks 5 days like I thought. I'm so relieved that at least there is a heart beat, but still nervous that baby's measuring smaller than I thought it should be. Any advice?