No baby on ultrasound just yolk sac
If you have an ultrasound and your doctor only sees a yolk sac and no baby it is probably just too early. An early ultrasound will not show a baby because the baby is too tiny to see and still a bunch of cells. Your doctor will probably have you come back in a week for another ultrasound to look for growth.
In the meantime try not to worry! You are probably just not as far along as you thought you were and it as too early for an ultrasound.