This isnt really about the dog, but I can hardly let the mention of the word D-O-G go by without saying something against them. I have had troubles with them, once losing an entire flock to normally mellow domestic hounds.
Most chicken people have some sort of canine troubles, sooner or later...
This often results from caponizing, too, and is called an air puff, or most often, a "wind puff."
As long as there is no other internal damage, you're alright there and aliena is likely right on track. If it bugs you, you can do what caponizers do: prick it with a pin to let the air out and...