No heartbeat?

Hey ladies, had an ultrasound done yesterday and i was measuring 6 weeks 3 days. I saw the baby and everything, but she couldnt find a heartbeat and scheduled me for another ultrasound next friday. She said usually they can find it but it's a gray area and can go either way. On my way out my doctor tells me if i start bleeding or spotting this week then it isnt a viable pregnancy. I have never spotted or anything so far and they have me a bit worried, has anyone else experienced this? I had a blighted ovum almost two years ago so this scares me.