What Causes Prolonged Vaginal Bleeding And Pain In Lower Pelvic Area?

Hi, i have been bleeding for about 3 and a half weeks now at first it was pregnancy then it happened i lost the baby miscarriage, and i have been experiencing some pain in my left side on the womb, i went to the doctor, when he checks the scan everything looks normal the ovaries and all but can t see as to why i am bleeding do you perhaps know why?