My sister never gains weight. recently she has gained weight and her stomach is very tight. she also hasnt had a period, her preg tests are negative. ?

Your sister may: have anorexia, which has associated amenorrhea (lack of menstruation), weight loss/failure to thrive, anxiety, insomnia, loss of hormonal balance, and malnutrition. She also may have primary amenorrhea or some endocrine dysfunction which is masking her weight problem. Please have her see a healthcare professional for help ASAP.

Unwanted weight gain: First distinguish fluid weight (like edema, ascites, "third spacing") from tissue weight (e.g. Bodybuilding, reparative, tumors, adipose). There are lots of metabolic reasons to rule out (organ disease like heart, liver, kidneys), insufficiency (thyroid, adrenal, kidney), more. Often high calorie (but low volume) foods & drink may add up quickly (3000-3500 extra calories per pound gained).