Im a 21 year old female that weighs 100 pounds and im 5' 3".. am i under weight?

BMI = 17.4: Ideal bmi is 20-25. That's ok, you could be a few pounds heavier, but it's better to be a little under than a little over at 21 yrs old. How do you feel? Do you have strength and stamina? If so enjoy your weight.

Yes, you seem to be.: Use a BMI calculator to know. SEE: https://www.nhlbi.nih.gov/health/educational/lose_wt/BMI/bmicalc.htm. BMI Categories: Underweight = <18.5 * Normal weight = 18.5–24.9 * Overweight = 25–29.9 * Obesity = BMI of 30 or greater. By checking your BMI you can get an idea of whether you are in a healthy range.