How much should someone who is 5'6 weigh? i weigh 128 and am constantly being told i am too thin but i do believe that to be the case. am i wrong?

Very individualized: and measures of muscle/bone/fat can be done in the office or heck... many fitness centers. 128 is not necessarily abnormal. A change up or down is generally more of an indicator of problems (if you were 170 and lost the weight in one month... bad).

BMI: If you use the BMI calculation, you are in the optimum range for a healthy 25 yo female. The BMI is a screening tool to identify possible weight problems. It is a measure of body fat based on height and weight that applies to adult men and women. Your BMI is approximately 21, which is excellent for you.