Perhaps he just ate something odd? There've been a few times when one of mine have had odd poops. Keep an eye on him and make sure he's got plenty of water to drink. If he's not acting odd (aside from that odd poop) I think he'd be ok. If he develops a fever, has really bad and repeated diarrhea, is listless or vomiting, I'd bring him to a vet.
If he's acting fine then I wouldn't worry about it. Just keep an eye on him and see how he does. If his stool starts to get runny and you notice he's not as energetic as usual, or starts vomiting, then I would call the Vet and make an appointment. As Ellen has said, it's possible he just ate something out of the ordinary.
Has anything changed with his food? Have you added anything to his diet? If it only has changed in color then I wouldn't worry but would watch it to see if it get runny or any other signs of something wrong. How old is he? Could he be teething?
If he is lethargic, vomiting, and has soft stool get him to the vet asap. Parvo has a very distinct smell so if this poo smelled REALLY bad and different from his other BMs and it continues to get softer then I'd take him in. Puppies are notorious for getting into things they shouldn't so he may have just ate something, also sometimes a lot of exercise will cause loose stool. I would take him in if it lasts longer than a day or two or if there is any vomiting or loss of appetite. Puppies get dehydrated really fast so you don't want it to go on too long. He may also have intestinal parasites, as almost all puppies are born with them and need to be wormed several times before they completely clear from their system.
Seanna's always gets looser when she exercises...to the point where it's like diarrhea by the time we're done. If it keeps up or gets worse, have the vet check him.
If you are worried about the stools - especially if it smells really bad- take a "fresh" poo sample to the vet if you haven't already. They can check for roundworm and other parasites that are really common in puppies. Parasites can make your puppy have loose stools and appear bloated. You'd probably have to pay an exam fee but the stool sample check is usually pretty inexpensive.
The vet can check for Parvo using a snap test. They will most likely check for parasites first and ask if your Corgi ingested anything out of the ordinary. My Charlie has constant tummy issues. I've had him for 1 year, and he has never had a solid stool. They think he has irritable bowel syndrome/ stress colitis, but they can't diagnose it for sure with out and Endoscopy, which is $2500 plus dollars I don't have.
I suspect your pup just got into something and got sick:0) Taking your pet into the vet is never a bad idea. I am the queen of overreacting when it comes to my pets. Charlie has been to the vet 25+ times in the past year. Every staff member at the vet know me and Charlie, even when I call, they know its me calling about Charlie.
Charlie was always active, drank water, played, everything a Corgi should be...except for the diarrhea. So diagnosing him was hard.
Charlie is the extreme case of tummy issues! I bet your pup is on the other end, less severe:0)