no heartbeat

All, I found out today at the doctor that I am measuring 6w2d. They were not able to find a heartbeat, they said this could be because I am early and not 100% sure on my first day of my last period (I am irregular). I was told I need to come back in for another ultrasound in 1 week and if no heartbeat there more than likely won't be one. I wouldn't say the nurse was "concerned" just more matter if fact. This is me and my husbands first child and I am not sure how to process all of this or what to think. Has anyone else ever experienced this? Does anyone have any insight?