I agree that you should see a doctor first. But, someone who knows about athletes and nutrition and can deal with your IBS, etc. For someone that is young and very physically active, that weight is not that low. But, the fact that your body is rebelling by having messed up periods, hair falling out is a warning sign that you aren't getting enough food or some nutrients! A lot of MDs do not know how to treat eating or nutrition issues...
I second the counseling and nutritionist. I went through the same thing as you when I was in my early thirties. I was not dieting and lost a great deal of weight from teaching aerobics. Everyone thought I had an eating disorder, but it was just the exercise. It's hard not to get caught up in thinking about food all of the time when you are getting compliments about how toned you look...
Of course, I still obsess about food, but so does everyone else in my family (we like to eat and cook, but not gain weight).