What Causes Extended Menstrual Bleeding And Cramps?

I am a 42 yr old female. Have always had menstrual cramps before monthly periods. However, periods have rarely been off schedule. My present menstural cycle is a huge exception. It has now lasted 3 full weeks with no sign of stopping. Help, should I be worried about a particular problem, if so, what. I am normally always anemic and have taken iron almost on a regular basis.
Currently, other than being very tired and the endless period, I have no other symptoms.
What could be the cause?!