Your menstrual cycle is under hormonal control. The ovary usually releases one egg (ovulation) each month, and about two weeks after, the uterine lining sheds, resulting in menstruation. If the egg is fertilized, it is implanted into the uterus and menstruation does not occur. However, if ovulation does not occur regularly, the body does not send signals to the uterus to regulate menstruation. Stress, jet lag, lack of sleep and too much exercise can disrupt your hormonal balance and cause a change in your usual menstrual cycle...consult nearest gynecologists they will do the needful.