The back of your head is right, 98 pounds at 5'7 isn't healthy. There are many illnesses and diseases associated with being overly skinny. I suggest you seek a psychiatrist or psychologist or guidance.
But, in the mean time, you can still gain weight, eat, and have a great physique. All you need to do is exercise. Make sure you eat at least 1,000 calories a day, it is EXTREMELY unhealthy to swoop below that, especially for a woman. I know girls who are your height, weight 120 pounds, and have a 24" waist. It's all muscle. They're healthy, and still very thin.
Good luck, and please seek guidance!