The healthtap bmi scale indicates that i was underweight. but my physician said that my weight is normal. what information should i rely on?

BMI and young people: BMI by itself is not that useful in younger people that are potentially still growing. For kids, we plot BMI against age and sex to get a percentile. HealthTap's BMI calculator is likely inaccurate. If your doc says you're good, you're probably good! But if you want, you can check out this CDC calculator that shows age-and sex-adjusted BMI percentile (http://1.usa.gov/1NAASb9)