I'm 16, and haven't had my period in 5 months. why? i haven't had sex

If you just: began having periods - they can be irregular for the first year or more. Reasons for missed / late period include: an anovulatory cycle in which no egg is released, marked v or ^ in weight; eating disorder; Over-exercising (endurance athletics), stress, travel, significant illnesses, oral contraceptives & other meds, Illegal drug use, hormone imbalances and medical problems etc. See gyn.