What are causes for evening diarrhea in puppies?

So this past 2 evenings my puppy has watery/pudding like poops. His other poops throughout the day are normal/firm stools. However, after his evening meal his poops are watery. Like I said this has been going on 2 days now. He is about 14-15 weeks old. No accidents inside, he holds in until outside and does have time to sniff around for about 2-3 minutes before he does poop. Puppy isn't throwing up. Energetic, drinking water. Any ideas?