As a reaction to the action of cool water a sort of chemical substance is released.
With this chemical substance the contracted nerves expand and transport more
blood to nerves and micro nerves. If the outflow of blood from the skin when we put
cold water is action, more flow of blood to the skin after a little while is reaction.
Here reaction is more than action. So the body is sending more blood to the skin.
The excess blood, which has come to fight coolness, remains in the skin for a long
period. We can feel our body warming up when we take cold bath. If we feel more
warmth after the bath it is because of more blood in the skin. The reason for asking
not to take bath after food is that the blood in the stomach that helps digestion
reaches the skin and remains there for a long time when we take bath. It means the
digestion will not be sufficient or it is delayed.