Only see a sac...

So I took a pregnancy test on March 1st and it came up positive right away. Last Friday I had my first appointment March 11th. I had an ultrasound after my appointment and the nurse called me a few hours later and said they could only see the sac and by that they were saying I'm 5w5d. The Dr called me Monday saying he wants an either ultrasound at my next appointment to make sure there is a baby and a heartbeat. As if today I'm 6w3d. Does this happen often? This us my first pregnancy and the phone call scared me.