Am I having a miscarriage?

I started bleeding a few days ago and went to the ER. They didn’t find any gestational sac yet and said that I am too early. My hCG was only at 481. They said that I need to wait for two days and get an ultrasound and hCG check with my OB/GYN. In the past two days I’ve been passing small clots and bleeding bright red. Has this happened to anyone and resulted in a healthy pregnancy. Or has this happened to anyone and resulted in a miscarriage. I have an ultrasound tomorrow morning.