What Causes Heavy Menstrual Bleeding Along With Lower Abdominal Pain?

After my delivery I got my periods after 10 months .i had heavy bleeding for 20 days then I have taken regestrone 5 mg tablet for one week bleeding was negligible later again it started I have abdominal pain also today is the 17 th day since iam taking this regestrone table . What next i have to do to stop this bleeding please suggest