What Causes Heavy And Foul Smelling Menstrual Discharge?

I am 46 yr am taking a number of medications that I started in August 2010 for a blood clot in my leg. I have always had a pretty regular period (every 28 days) however, this month my period started yesterday, a full week early and the flow is darker in color and has a foul smell.
what could be causing the smell?