17 and 165 pounds, my period missed a month and i'm not sexually active.

Irregular periods: Irregular periods are not unusual in your age group. See your doctor if missed periods persist despite not being sexually active.

Common at your age.: Reasons for missed /late periods: low body weight, obesity, marked weight gain or loss, over-exercise such as in endurance athletes, breast-feeding, stress, illness, eating disorders (bulimia or anorexia), travel, hormone irregularities, drug use, medications (i.e., birth control) or med problem (polycystic ovary syndrome or Asherman’s syndrome. #1 reason is pregnancy.