Emergency Ultrasound

Yesterday at 9 weeks and 1 day I bled really bad, right through my pants and everything while I was at work. Went to the hospital and everything, had the ultrasound about an hour ago. So relieved to hear the babies heartbeat is at 170 bps and growing at the perfect rate. I just want to know why I have been bleeding for four weeks off and on from spotting to red bleeding, I guess I have to wait till Monday for my results.