You most likely ovulated late. Or the baby stopped growing. But I would hope it was the first one. ;) Don't worry about it too much. The 21st isn't so far away and then you will have your answers. Have faith in your baby.
When we went for our first ultrasound we were expecting to see the fetal pole and heartbeat, but only saw a gestational sac and BARELY a yolk sac. It was stressful, but it just turned out that our dates were off by about a week and when we went back two weeks later there was a heartbeat and it was amazing. :)