What Causes Clear Blood Discharge From Vagina After Completing Menstrual Cycle?

Hi i am having alot of trouble with my menstual cycle, i used to be very regular, and they used to last about 1 week, but in the past few years i started to get alot of clear discharge after my cycle was over, then it changed colour, and lasted longer, now i get it for nearly two weeks after my cycle and it looks like blood,and smells like iron, i have been tested by my doctor several times, but they cant find a cause, im 36 years old, what can be causing this?