Bleeding and slight cramping

I went to the hospital Tuesday night because I had heavy bleeding but I wasn't cramping at all. I was relieved when they did an ultrasound and the baby's heart rate was 120. I continued to bleed and went to my doctor today he sent me to another ultrasound and i'm 6 weeks 4 days and the heart rate is 125. They don't kno exactly what's causing me to bleed so heavy but baby is perfectly healthy.