Spiciness isn’t a flavour, not like sweet or sour - it’s just the result of the activation of pain receptors in the body. The chemical that causes the spiciness in chilli peppers is capsaicin, which is what creates the burning sensation when eaten.
Your body actually views capsaicin as an offensive substance which needs to be immediately flushed out. Capsaicin latches on to pain receptors in the nose, mouth, and skin, which are normally only activated in the presence of heat. It irritates the mucous membranes in the nose, causing them to be inflamed.
Nutritionist Melissa Calendar explains “capsaicin irritates your mucus membranes, especially the ones in your nose, so as a defense mechanism they produce more mucus, causing your nose to run so the extra amounts of mucous are actually a defense mechanism to try and keep out the unwanted substance.”
The sensation produced by the capsaicin is the same sensation that heat would cause, which explains the burn. The capsaicin tricks the nerves and sends messages to your brain. This signal turns the nerve cell on to allow it to trigger other nerve cells that will carry the message to the brain that it has to respond to this dangerous temperature - hence the sweating that often accompanies a spicy meal - this is the body's reaction to try and cool itself down.
So why the red face and hands? The redness on your hands and face is another sign of your body attempting to cool off. Capillaries below the skin dilate in response to the “heat,” and blood rushes through them to move heat to the surface of your body, where it can more easily radiate away.