I'm a 19year old female and I am 5 ft 3 and weigh 90 pounds. I cannot gain any weight. I eat a well balanced diet. What can I do?

Weight. I would go to gnc or the like and purchase a weight gain product. These are packed with calories and protein so be sure to drink extra water during the day. When you reach the weight you want cut back on the product, eat normally and find the amount you need to maintain the wt. I know many people who would love your problem, good luck.