Golden retriever pup with stomach problems

[Raivotar]

Hello all,

I've been doing some research on this issue for a few days and I see a lot of conflicting stuff. Here's the rundown:

I have a 15 week old golden retriever pup who is having a lot of loose stools during the night/early morning. During the day, his stools seem to be fine. This has been happening for almost a week now. I haven't changed his food or given him anything out of the ordinary. Two days ago, I didn't give him any treats and just fed him his regular meals and he was fine. Yesterday, I gave him a few treats throughout the day and he had problems again at night. Tonight, I just fed him his regular meals with no treats and it happened again. I thought maybe the treats were the problem, but I guess not.

He's not vomiting and he doesn't seem to be fatigued or acting any differently. I took him to the vet just a few weeks ago and the vet said he was healthy and growing just fine. I'm going to drop off a stool sample to the vet later today for testing. My question is this; why would it be happening only at night and not every day? Could his food be the problem or how many times a day he is fed? He's been having the same food since he was 7 weeks old (Iams puppy, which I'd like to change and give him better quality food), but could this just happen after a while even if he is used to the food?

He is crated at night and has been since I got him. He has adapted very well to his crate, but could this be some sort of developing anxiety? I would imagine that if he were sick, this would be happening all of the time and not just at night, but I don't know. I'm really, really stumped on this and any kind of information or tips would be greatly appreciated.

Also, I don't know if it's worth mentioning, but when my pup eats his meals, he eats really fast. I know personally, when I eat fast, I don't feel so great afterward, but I see dogs eat fast all of the time so I didn't think anything of it...