Measuring 2 weeks early and low heart beat

I went in at 8 weeks for my first ultrasound last week. They informed me they were having a hard time hearing a heart beat but they see the flickering of heart beat which seems slow. They also informed me the baby was measuring two weeks early. I find this strange as well because I got a positive test at four weeks and I am very regular. We are waiting till this Thursday to see if it is viable now. I have no miscarriage symptoms so far and no morning sickness. I didn't have morning sickness with my first baby though either. I am very sad and nervous and hoping for the best. it is upsetting to be illusioned by this idea for 8 weeks and now find out that I might not have this baby. The tech says she has seen this many times before where the mom comes back and every is progressing fine with fast heartbeat.