6 weeks 4 days instead of 8 weeks 5 days

I just had my first ultrasound, and the tech said that I'm at 6 weeks 4 days, instead of 8 weeks 5 days (which was the beginning of my last cycle). Should I be worried? The babies heartbeat is at 115. I took my first home pregnancy test 3 weeks ago and it was positive, with pregnancy symptoms starting at least a week before that. I'm worried that my baby isn't growing :(