heartbeat but no yolk sack

Mi
Hi, I am 6w 5d. Had my first ultrasound last night and the baby had a heart rate of 122bpm and all looked good. 
I got called into my doctor urgently though because the sonographer wasn't able to detect a yolk sack. 
Has anyone ever had this happen and had a positive result?? I have hardly any pregnancy symptoms apart from some strong on and off cramps the past couple of days. Going back in a week for another ultra sound.