text: When you're in water, aka a bathtub, or in a pool, your body senses water, so your cervix closes up making blood not come out anymore.  The reason why people wear tampons when they go swimming isn't because of the actual time in the water, but when you get out of the water, you'd have to make a mad dash to the bathroom because your body responds quickly to you not being in water.  I'm guessing while you're in the shower it's the same way.  I've also noticed that while I shower and I don't have my period it doesn't seem to drip.