You don't say how old you are, how much you weigh or whether you are having symptoms of menopause (hot flashes, vaginal dryness, etc.)
There are many things that affect menstrual cycles. If your body fat content is below a certain percentage, you will stop ovulating. Prolactin is the hormone secreted by the pituitary that suppresses the cycles while nursing. That is sometimes overexpressed by a pituitary tumor, causing amenorrhea. Another cause is lack of progesterone at a certain point in the cycle. Normal menopause may hit as early as the late 30's.
I strongly encourage you to have your gyn do a hormonal workup. Endometrial cancer can result from lack of menstrual cycles while still ovulating. Once you understand your particular hormonal situation, you can think about how to deal with it. Just don't ignore it!