Wrong dating in pregnancy ultrasound.

So i went to a clinic and a pregnancy test done on March 10th that was negative but i just knew i was pregnant so i took a home test on March 13th that came back positive.So I went in for my first OB appt after a positive pregnancy test. I thought I was about 6 weeks and 4 days pregnant from the day of my last menstrual cycle. I expected to see some sort of my baby, however when they did the ultrasound all they could see was a gestational sac. Nothing else. They say the believe I'm only around 3 weeks pregnant. I don't know how that is possible but I'm concerned now. They scheduled another ultrasound for April 6. I'm hoping I see some sign of my baby. I'm so nervous. What should I be seeing? What can I expect to see at my next ultrasound? Please ease my mind.