Enteritis.

This is due to inflammation   of the bowels and can be the result of many different causes.  The dog might have eaten something unusual which could have caused the problem.  It could also have been caused by worms.   If the cause is worms, this is often easy to determine by examination of the stools.  In certain instance however, this is not always a completely reliable test.   There are however, excellent worm remedies on the market that have the effect of ridding your dog of the common types of worms.