It definitely sounds like something is not quite right (re. the periods, hair falling out, etc.), but unlike some who have posted, I don't find your weight to be alarmingly low. I am 5'2" and 103-104 lbs with tiny bones (bird wrists, small ankles, narrow hips) and am never told that I am too thin. I'm actually curvy, and in fact, now that I'm in my mid 40s, I've got quite a bit to pinch around the middle and "love handle" area and am trying to gain more muscle, because it's certainly possible to weigh relatively little and still have a higher body fat % than ideal. I've been this weight for years, am very active, don't count calories, eat what I want, almost never even catch a cold, and have very normal periods.

So, while I'm not at all alarmed by your weight, the other health issues do suggest that something is amiss. With the diet you quoted, you certainly need more calories since you're riding, being active, and are young so naturally have a high metabolism. I don't count calories, as I said, but I'm quite sure I'm eating ~1600-2000 calories a day, and more on the days I work out intensely. One way to get more calories in is to eat SNACKS! Our mothers may have told us they are bad for us, but healthy snacks are just that -- healthy! I eat fruit, nuts, yogurt, etc. for healthy snacks, several per day. I have a fast metabolism for my age so end up eating every couple of hours. I eat small to moderate healthy meals (I avoid restaurant meals, fried foods, etc.), so snacks are a must to keep up energy and keep hunger at bay.

Good luck and please keep us posted!