Hi.. Irregular periods are known to occur due to various reasons.. It is especially very common in young girls as the hormones rhythm is still not completely normal... These hormonal causes may be the reason. However other conditions such as lack of proper nutrition, stress, infections, anaemia, endocrine abnormalities, PCOD are also known to cause menstrual irregularities.. I would guess it is more due to the immaturity in the hormonal rhythm and mal nurishment as the probable reason cause for your irregularities.. Reduce your apprehension and stress as they will only make your symptoms worse.. Check with your doctor..