How can i be underweight but not have an eating disorder?

See your doctor!: 1) burning too many calories. 2) not eating enough calories. 3) not absorbing the calories you eat: examples of disease processes include: small bowel malabsorption, pancreatic insufficiency, inadequate calorie intake, poverty, malignancy and chronic inflammatory conditions (cancer, crohn's disease, celiac sprue), parasites (tapeworms).

Good observation...: Looks like u're trying to lose weight & u're noticing that ur ways are suggestive of an eating disorder. The focus now should be on the eating disorder & it's underlying belief system, body image, related issues... U'll then be freer to lose weight (or not) from a space of wellness & wholeness, not a space of confusion & lack. Ur goal will also likely become to lose weight, not be underweight!