Answer Digestion requires good blood flow and digestion generates heat. If one takes a warm shower, does exercise or goes for a swim, the blood needs to be diverted to the skin surface for cooling purposes. This interferes with digestive process and you can end up having a stomach ache, can faint or even have a heart failure if you are weak.
Though most people would see no ill effects having showered right after a meal, but weak or old people can have a different reaction. So a healthy habit is to wait for about 30 mins after a meal before taking a shower.