I am menstruating frequently, ie. every 2 weeks. what is the cause?

Perimenopause: The extra bleeding, especially if occurring in a predictable pattern, could be associated with ovulation. There is a small decrease in estrogen at that time. As we age we make less estrogen and the lining of the uterus can become unstable and cause spotting. But you could have endometrial polyps or hyperplasia. I would recommend seeing your gyn. Expect an ultrasound and an endometrial biopsy.