Puppy has diarrhea. What could be wrong?

I have a puppy (boxer mix) who is, roughly, 4-5 months old. He has had 2 of 3 of his Parvo etc shots and is due for his next round soon. He eats Diamond Puppy Food and has since I got him. His bags have NOT been included in the voluntary recall.

Recently, I have noticed something that concerns me: in the morning, my puppy's stool is soft and runny (pudding consistency,) But only in the morning. As the day goes on, my puppy's movements are solid and normal. This is the third morning where this has happened and I plan on seeing a vet TODAY. However, I am curious to know before I go in what could be wrong.

His behavior is completely normal. He is eating regularly, drinking regularly (perhaps a little more than usual since I make sure to keep him hydrated since I know how quickly diarrhea can dehydrate a puppy,) and playing regularly. He seems to be in absolutely NO discomfort - right after diarrhea in the morning he will bound out and about in the yard playing tag with my other dog.

I do not suspect Parvo - as I have said, he is not behind on shots and we have not taken him ANYWHERE outside of our backyard because he has not had all of his rounds.

I should note that my puppy does have a habit of getting into things he shouldn't - he loves to eat my shoes and I caught him eating a pair the night before last. But after eating my shoes the other day I cannot think of anything he might have gotten into.

When I took him outside for the last time last night (11:45 or so, PM) his stool was solid, This morning I took him out at 6 AM and it was pudding consistency. This has been on-going for 3 days now (today is the third morning where this has happened) and I will reiterate that we are seeing the vet this afternoon.

Any ideas?