Why does erythromycin cause severe stomach pain in some people? what did the drug do in these cases?

Erythromycin: ... Can cause severe stomach cramping at times, since it increases the motility of the digestive tract. Although painful, the resulting spasms will not cause any long-term damage to your digestive tract. Take it as prescribed, for the full course, but if it continues to give you too much problem, they contact the doctor who prescribed it to see if there is an alternative. Good luck.