While some women do have low weights because of metabolic issues, it sounds to me like you very well may have an ED. 5' 8" and 100 lb. is very, very underweight. In fact, it's dangerous to your health to be so underweight.
One thing with my ED, I always made excuses not to eat. I'm busy studying, I'm in college, don't have time for balanced meals, can't find time to eat, I'm too depressed to eat and so on. It sounds to me like you're making excuses and are in denial about having an ED.
Read the signs of an ED in the sticky post up above. One major red flag that you probably have an ED is that the thought of gaining weight, even 1 lb. terrifies you. That you still feel fat or not thin enough as thin as you are.
I suggest seeing a therapist (preferably one specialized in treating EDs) and a psychiatrist and a registered dietician. It sounds to me like you do have an ED but just keep denying it (even to yourself).
