Keep in mind that there are a lot of things that can affect your cycle and your flow -- though the shower itself probably isn't one of them. Try not to worry about whether this sudden stoppage is normal because when it comes to periods, what's typical for you may be different from what others experience. It could simply be a coincidence that the flow stopped just as you stepped out of the shower. (Maybe it even started up again later?) Some days, you might have a heavy flow in the morning but lighter in the afternoon. And, of course, it takes a while for your periods to become regular if you've just recently had your first one. So try to be patient. In time, you'll probably see some patterns and become more regular. Although I have to say, it would be kind of nice to be able to just step into the shower to stop the bleeding sometimes, wouldn't it?