Strenuous exercise should be avoided during menstruation, so you should not run after your period. The purpose of paying attention to avoid strenuous exercise is: first, to consider that the amount of vaginal bleeding will increase after strenuous exercise during menstruation, leading to blood loss and anemia; second, it may prolong the menstrual period and increase the chance of infection; third, and the most important factor, it may increase the risk rate of endometriosis. Because of strenuous exercise, the uterine blood vessels open during the shedding of the endometrium, which may cause the endometrium to become ectopic to the myometrium, resulting in adenomyosis, or an increased risk rate of endometriosis later. Therefore, strenuous exercise, especially running, should be avoided during the period of menstruation. If running is necessary you should also ask for leave from your teacher. Self-developed physical exercise should be stopped during menstruation.