Why might i look fat if my bmi says i'm underweight?

Two issues: Bmi is an estimate dependent on a fudge factor that assumes bone structure & muscle mass are consistent along all body types.It can be off some. To verify bmi assessment, plotting height & weight along standardized growth charts (cdc or who) gives a better graphic representation of proper weight for height. If you are underweight by both systems, your brain may be giving you a distorted self image.

Distorted Body Image: BMI from 20 to 25 is in the normal range. If you are falling into this range and are still feeling like you look fat this makes me concerned about you may have some tendencies of anorexia nervosa. We live in a culture which blasts thin models at every turn. Most of these gals are severely underweight. See a psychologist for starters to explore these issues!