Here is another website to look at lil-BBerry.
http://www.youngwomenshealth.org/triad.html
There are some good ideas in that website including lessening the amount of exercise you do, eating more and seeing a doctor. I think that you should seriously consider seeing a physician -- as I mentioned, a Planned Parenthood Center will give you a good check-up for not very much money.
Find A PPC Healthcenter:
http://www.plannedparenthood.org/pp2/portal/healthservices/findhealthcenter/
Although you don't mention the portions, it appears that you are not getting enough protein in your diet, as well as not enough fat.
Don't eat low-fat yogurt... eat regular yogurt.
Drink 2% or whole milk.
Add a banana to your breakfast and a piece of toast with butter.
Be sure your cereal is enriched and nutrient-rich. A very good kind is made by Kashi. Eat a double-serving.
Try to eat an apple with every meal.
Add more fruit and vegetables to your diet.
Add more meat to your sandwich and to your dinner.
Add beans to the rice with your dinner.
Occasionally eat ice cream or real jello instead of something sugar & fat-free.
Do at least a few of those suggestions for a week and see if you haven't gained a pound or two -- with the ultimate goal of gaining five to ten pounds. The Body Mass Index is a gross method of determining health, but it definitely points to your being underweight. That is the most likely cause of your amenorrhea (that means missing periods).
http://www.wrongdiagnosis.com/a/amenorrhea/underly.htm
List of symptoms of Underweight: The list of symptoms mentioned in various sources for Underweight includes:
Low body weight
BMI < 20
Vulnerable to infection
Loss of menstruation
I'd like to hear how you're doing and if you've gone to a clinic. Be well.
Piffka