Loose or watery stool can follow a change of food, eating something unsuitable, or infection, parasites, inflammation and other problems in the gastrointestinal tract. Many mild, short-lived episodes pass, but the look of the stool is rarely enough to tell what is causing it.
Keep track of how often your dog needs to go out, whether it can drink normally and whether its appetite and energy are still there. Also note whether there is fresh blood, whether the stool turns black and tarry, or whether your dog is vomiting at the same time. Puppies, older dogs and dogs with other illnesses can be affected by fluid loss more quickly than a healthy adult dog.
Contact a vet if the diarrhoea is severe or persistent, or if your dog becomes lethargic, is in pain, vomits repeatedly, does not want to drink or there is an obvious amount of blood. With recurring problems, tests may be needed to tell a temporary stomach upset apart from an underlying disease.