https://pets.webmd.com/dogs/6-most-common-dog-health-problems#3-7

Diarrhea in dogs, as with vomiting, can have lots of causes, including stress, infections like parvo virus, intestinal parasites, and food problems.

Diarrhea symptoms are pretty obvious — look for loose, watery, or liquid stool.

Because diarrhea can easily lead to dehydration, be sure your dog has plenty of clean water available, then take your pooch to the vet if the diarrhea persists for more than a day, or immediately if there’s also fever, lethargy, vomiting, dark or bloody stools, or loss of appetite.

WebMD Veterinary Reference Reviewed by Amy Flowers, DVM on September 10, 2018