What Causes Malodorous And Dark Coloured Menstrual Blood?

I am 38 and for the first time since my periods commenced at age 15 I have foul smelling menstrual blood. The smell is so bad that it is making me feel nauseous to change my tampons. The blood loss is the same amount but is a little darker in clour than usual. I don't seem to have any other symptoms. Help!