A: It doesnt actually stop; the lining of your uterus continues to shed. But the counterpressure of the water can stop the flow from coming out of your body. Oh, and the temperature of the water has nothing to do with it—this can happen whether youre in a freezing-cold lake or warm Caribbean waters. Plus, if youre floating or swimming horizontally, gravity isnt able to pull the flow down and out as well as it would if you were in a more vertical position. (The same thing can happen when youre lying down.) Once youre out of the water and standing upright for several minutes, your period should return to normal.