First ultrasound - experiences

For those who've had an early ultrasound, like 6ish weeks and below;  what have your experiences been with what you can see?
​I had my first u/s yesterday thinking I was 6w1 from my nurses calculations. They measured it at 5w3. They also did not see a yolk sac, only the gestational sac was there. I know it's early, but in still concerned. Has anyone else had this happen?