Your puppy could either have an abscess (pus and fluid) or a hemotoma (collection of blood). In either case, your pup needs to be examined by a vet. The vet may drain the fluid and/or give medicine to fight infection.
Take him to the vet immediately. It sounds serious and may be an infection of some sort, or a build up from something. Call emergency vets if its after hours.
That sounds disgusting! I certainly hope its not fluid, but if it is, the puppy could have been bitten by an insect or it may have an allergic reaction. Take that poop puppy to the vet!
