My dog has blood in his diarrhea?

My 9 month old yellow Labrador was neutered a few days ago. I noticed bloody diarrhea this morning (a Sunday morning) and am not sure of the severity of his situation. We gave him a rawhide bone yesterday evening which we noticed later on was yielding small shards of rawhide. The blood is bright red, there are no visible lacerations to the rectum. He seems to stay hunched for long periods of time and often, as if trying to p something or as if his stomach is bothering him with minimal display of discomfort. What should I do??