I might have been thinking of atypical anorexia/something similar, but if I'm remembering the diagnosis criteria correctly, anorexia is a major restriction of food intake (beyond normal dieting), so if you had someone at 5'4, 115lbs, but they were only eating 300 calories a day and losing weight fast, that could be anorexia. It is true that it couldn't go on for very long before they become underweight though, so that would be a very early-stages diagnosis or a relapse after gaining weight in recovery.