Black and Watery Puppy Stool?

We just bought a 4 month old dudley yellow lab from a farm here in Illinois. We've had him for almost 4 days now. He has been gassy the entire time and has been drinking excessive amounts of water but seems very lively and happy. Starting today though, his stool has become extremely watery and black. I am worried that he may be sick or that he has swallowed a foreign object etc. We feed him high protein, big breed puppy chow and my mother things that that is the cause but I'm not so optimistic.