I had another u/s and baby is doing well heartbeat and all now!
It's pretty early, most times you won't see much on an ultrasound until 7wks.
Are your levels rising correctly? Doubling?
Has your doctor told you anything? They should advise you to have another ultrasound in 2 weeks time too see if any growth has happened.
It is possible that everything is ok but you won't know for sure until your next ultrasound.