What Causes Vaginal Bleeding During Early Pregnancy?

Iam six weeks pregnant. I did scan which showed only gestational sac. No yolk sac & fetal pole. After two days I started bleeding. Doctor checked and she said it is moderate bleeding & prescribed ethamsylate 250 mg tid for five daya & told me to come back for rescan. I am still bleeding with litter clots, please advice iam lossing my baby??