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. 

Glow Resources

Let’s Glow

Glow is here for you on your path to pregnancy

Glow helps you navigate your fertility journey with smart tools, personalized insights, and guidance from medical experts who understand what matters most.

25+ million

Users

4.8 stars

200k+ app ratings

20+

Medical advisors