ultrasound at 6w

I'd been having some cramping on the left side, and they had me come in to make sure it wasn't ectopic. I thought I was 6 weeks 2 days and was hoping to see the baby and heartbeat. They only saw the gestational sac and yolk sac and said im probably more like 5 weeks 4 days. Im going back for another US in ten days. Anyone else experience anything like this? Im freaking out and so nervous. My doctor didn't seem at all worried and said that my blood levels and the sacs looked great.