Stressing out please help !!!

So according to glow I'm 6 weeks I went to the dr this past Tuesday and had an ultrasound and the dr said there is definitely a pregnancy but they are going to take my blood every few days to make sure my hcg level goes up and not down . Is this because he didn't see a heartbeat? I know it's early still but aren't you suppose to see a little Tiney dot or something ? All I saw was a circle he said was the sack I think . Help please