When puppies go to their new homes the change can sometimes cause them enough stress for the bacteria to emerge and will cause diareah. (cruisincavaliers.com)
If your puppy / dog has diareah for longer than 24 hours and you have not changed their food then I recommend either taking your dog to the vet so your vet can retrieve a stool sample or you can use a zip - lock bag... turn the bag inside out and use it like a glove to pick up a sample then carefully turn the bag the correct way and zip it shut. (cruisincavaliers.com)
If this is the cause of the diareah your vet will also need your dog's weight in order to prescribe a liquid medication called Albon. (cruisincavaliers.com)