It sounds like hormone and thyroid testing may be a good bet for you. It may be that you are chronically low in one area or another and that it's causing your amenorrhea. It doesn't seem that you are exercising too strenuously, and nothing you are taking would seem to cause this.
You might consider going to a naturopathic physician also. Naturopathic physicians study the entire body (not just the symptoms in one area) and use nutrition, exercise, lifestyle changes and natural healing methods to help a body restore itself to proper function. Here's a page from the American Association of Naturopathic Physicians that explains, adds links and lets you search for those near you:
So maybe an endocrinologist and/or a naturopathic physician? I'd be interested to know what those hormone and thyroid tests tell you and your doctors.