7 week ultrasound no heartbeat

My last period was December 20, 2017. Counting from that date I should be 7 weeks pregnant. Went to have my ultrasound yesterday and there was no heartbeat and the embryo was measuring at 6 weeks. I know I could have ovulated later then normal but I took my pregnancy test and got a positive on January 19, 2018. Even if I would have ovulated a week later than normal could I get a positive pregnancy test that early. Please I need some answers. Took a blood test yesterday going for another tomorrow to see if my levels double and going for another ultrasound next Thursday. I'm so nervous I'm afraid I had a miscarriage. it is all I can think about.

UPADTE: Unfortunately I did end up having a miscarriage. This has been one of the hardest things in my life. My fiance and I are doing our best to get through it. We are hoping to get pregnant and have a successful pregnancy soon.