most common causes of irregular periods: Eating disorders, like anorexia and bulimia Excessive weight gain or weight loss; extreme thinness and obesity both cause your menstrual cycle to become irregular or even to disappear Stress or emotional problems Hormonal problems, for example, when the thyroid malfunctions Travel Over-exercising: If you re an athlete and train very hard, you may stop menstruating because your body wants to survive and save energy and menstruation requires energy Problems with the pelvic organs (such as polycystic ovarian syndrome, for example) Drugs like birth-control pills can affect the frequency and / or intensity of menstruation Breastfeeding. I hope you happy with information kindly rate it. thank you .