Digestion requires blood flow and digestion generates heat. If you take a warm shower, exercise or go for a swim, your blood needs to be diverted to your skin surface for cooling purposes. This interferes with your digestive process and you can end up having a stomach ache, faint or even have a heart failure in some cases if you are weak. Most people would see no ill effects showering after having a meal
It’s better to wait for about 30 mins after a meal before taking a shower.
The reason you might faint is that there won’t be enough blood flowing to the brain. Digestion requires a high percentage of blood and when you have your warm shower the blood vessels dilate in order to cool down the body by releasing the heat from blood to the skin which means more blood flows through those vessels and since there will be so much blood being used elsewhere your brain might lack blood hence the faint part.