My younger sister is like you. She's 5'9" and 125 lbs, so she's right at the lower end of the BMI scale (18.5). She's the type of person who doesn't eat very much when she gets stressed, and school just started so she's pretty stressed out. My mom keeps giving her grief because she thinks my sister is anorexic - unfortunately my mom doesn't know what that means, she seems to think that anyone who is already thin and then loses more weight is anorexic. Surprisingly, she's not fat.