Your stomach lining turns red when you blush, a response that is the result of the sympathetic nervous system causing increased blood flow throughout the body. During periods of embarrassment, the body releases adrenaline, a hormone that prepares the body for stressful situations. Adrenaline also makes the blood vessels dilate in order to improve blood flow and oxygen delivery in case the body has to suddenly flee a dangerous situation. The veins in the face, as well as in the stomach lining and throughout the rest of the body, then have more blood than normal flowing through them, and they appear red.
More about blushing: