You need to get the puppy to the vet ASAP. It could be juvenile cellulitis, also known as puppy strangles.
Puppy strangles only affects puppies under about four or five months of age. Usually the treatment is a combination of prednisone and an antibiotic, but it's important to get this diagnosed because you can lose the puppy if you do not treat it.
Please let us know what the vet says and also how the puppy is doing.
Ghilly
There is no way to tell you what that might be. Puppies can go down hill very quickly. I strongly suggest you have this checked by a Vet ASAP!