Diarrhea is a great indicator of there being a problem. It also can happen because of stress or other minor issues like an abrupt change in food. It can indicate parasites. Diarrhea can be the first sign of many serious problems. You can try a little canned pumpkin with food and even 'share' a little plain yogurt in case some good bacteria is needed but if for more than 3 days, severe, or if there are ANY (vomiting, if the stools are bloody, black, any other color or mucus is apparent or any other abnormal consistency or something you notice about it, fever, lethargy, depression, disorientation, even a small change in eating or drinking habits, pale gums, yellow gums or eye whites, dehydration, any changes in normal behavior even very minor) other signs or if you gave your dog some over the counter ANYTHING you should get to the vet as soon as possible. Sometimes it can make all the difference.
