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this is Dr. Levinson,
I'd still get him checked out even though he doesn't LOOK like anything is wrong with him. His adrenaline could be kicked in so that he doesn't feel any pain, just yet.
the swelling could be from a crushing incident like being ran over. The fluid could be cellulitis which with time should resolve but it could also be blood pooling under the skin.
to be on the safe side i'd still get him checked out by a vet if not tonight then definitely by tomorrow morning.
just wondering how your pup was doing?