My sister thinks she's fat, is she?

I have a little sister who's too young to have Yahoo! ANSWERS so she wants me to ask. She's 11, and 4'8. She's 80 pounds, a BMI (Body Mass Index) of 17.9, but she's a little more than 50 percentile. Some of her friends tease her, and she's really upset. She gained 5 pounds in less than a week. So is she fat? Also she's constipated, so is that why she has gained some weight?