Opiates will do that, including Morphine. But what the heck, you're in a hospital! Have you told your nurses this? They should be able to give you something to help that along. Even just prune juice and/or raisins. But there's other means as well of course, that are right there in their hands.

If you haven't yet told them, please do. That will just cause you unnecessary discomfort. :\