I'm very sorry to hear about the loss of your pup. The first thing that comes to mind would be Parvo or distemper...both of which are contagious diseases that are most prevalent in unvaccinated younger dogs and pups.
You can find more info here:
Of course, it could have been something completely different, but the fact that the pup was so young and most likely not completely vaccinated makes me think it was one of the two above.
I hope this helps.
wouldn't he have had a temp or diahrrea? He was constipated and did not have any blood in his stool or anything. He died less than 32 hours after he got sick. He was completely normal the day before.
Generally yes, there's a temp and/or diarrhea, however I've seen puppies with Parvo who acted and looked just fine.
Unfortunately, it could have been a genetic flaw that caused your pups death as well, but there's truly no way of knowing what caused his death without a necropsy.