What Causes Heavy Bleeding After Miscarriage?

hi i m neha 27 yrs old it was my first pregnency and i got miscarriage after six weaks so i gt an abortion through pill by dr/.dr. said that bleeding will continue 7-15 days but now its 24 days and i m still having bleeding but its very light now seems to be over now.....why it happened is there is any risk....although on the 23 day i gt an ultrasound and dr. didnt fnd any pregnency tissue...nw which precautions i shud tk.