Yolk Sac but no Baby

Samantha
Hello.
Has this happened to anyone else. 
We thought we were almost out of our 6th week due to my LMP. But when I went in for my ultrasound yesterday there was only a gestational sack and a yolk sack. My doctor said we may be a week off and that I was closer to the end of my 5th week. 
I thought I knew when I ovulated but it is possible I was wrong. But based on when we did the baby dance it doesn't really make sense. But my doctor didn't seem to be worried and scheduled me for another ultrasound in 3 weeks. Which feels like forever when you now have the question of if your baby is developing correctly. 
I don't have a picture per in the moment I didn't think to take one, and my doctor didn't print one out. But it was a clear gestational Sac and clear round yolk Sac.