With a pup this young I would always worry about an obstruction of the bowel from a non food item, or that the dog may have gotten into spoiled food or a toxin. Check the area where the pup frequents and make sure there is nothing amiss. Look for overturned trash etc. If the pup got into something spoiled (meat etc.) then you may be looking at pancreatitis. With this there would be abdominal pain, vomiting, diarrhea, fever and lethargy.
If the pup eats non food items such as raw hides, bones, toys, sticks etc. then I would worry about an obstruction. Symptoms of this are sudden lack of appetite, vomiting, diarrhea (or off color/different consistency stool) and lethargy. This would require surgery.
Obstruction (foreign body)