Diarrhea may occur occasionally throughout a pet's lifetime, due to dogs and cats (dogs especially) tending to ingest sometimes less than ideal items. Diarrhea due to this type of dietary indescretion, may respond by feeding a bland, easily digestible diet. If you call your vet's office, they may dispense a prescription GI diet, to feed your pet until the diarrhea resolves. Transient diarrhea of this nature should last only one to a few days, should not be severe in nature or have blood in it, and not be accompanied by complete loss of appetite, vomiting, depression, or pain.
However, diarrhea could indicate more serious underlying causes, as well as put the patient in danger of dehydration and electrolyte abnormalities - as such, advanced diarrhea of this nature warrants a visit to the vet ASAP.