supposed t be 8 weeks , measured 6 weeks 3 days

I am absolutely paranoid now!! We've been trying for two years and now I'm pregnant!! But I went in for my first ultrasound and the baby measures only 6 weeks 3 days. There was a great heart beat at 122 and she could see it both TV and through my belly... But I'm so confused!! 2 weeks sounds like too many days to be off!! And when I did my HCG levels on Jan 3 rd they were right along between 5-6 weeks like we thought according to my last period!!!