Dark Colored Bleeding Not Stopping After Regular Period, Severe Back Pains. Should I Be Concerned?

Hello

My menstrual cycles after having my two year daughter has been consistent and usually last 7 days. My last period started July 31, and after 6 days of regular bleeding on the 7th day I had the darker (black looking) blood that usually indicates my cycle is ending. The problem is that sort of bleeding has not stopped. It was even quite heavy on some days and yesterday I noticed it was looking pink almost red again. And today I'm bleeding as though it was the first day of my period. I have been having sever back pains along with the bleeding. Is this something I should be concerned with? Any idea what it could be? I am not on birth control or sexually active for at least 4 months now.