Any number of reasons. Hormone fluctuations caused by stress, diet, exercise, lack of sleep, or just the whole very complicated interaction of all the hormones, or the menstrual flow is temporarily blocked at the cervix from a clump of uterine tissue, or you have a flopped uterus - so many reasons, not to worry, very common. You worry when there are really drastic changes over a long period. For instance, having a period that lasts for weeks might be related to the thyroid, or painful cramps might be fibroids.