Menses are mostly irregular. And this can happen even in normal women as the levels of hormone keep changing every cycle and are influenced by many things like thoughts, emotions, weight, etc Menses means the expulsion of the lining of the uterus. It happens periodically. But there is NO fixed cycle length from one menses to another. It can vary from 21days to 35 days and even 45 days in some. If the lining of the uterus is not formed upto normal levels the flow will be reduced. If it is formed more, the flow will exceed. Hence do not despair. If you are worried you can consult a Gynecologist who may advice some tests like thyroid test and ultrasound to rule out diseases of Thyroid gland and ovarian cysts/