What is the exact mechanism of paralytic ileus post-operatively.?

Common. This is a normal or common reaction to the trauma of surgery, anesthetics, and narcotic pain medication. These all cause the muscular wall of the small intestine to essentially fall asleep and stop functioning. This then results in temporary build up of air and fluid inside the intestines. As the intestinal wall awakens, abdominal cramping may occur, then they gradually return to normal function.