My 5 y.o. Jack Russell Terrier (runt, about 8-10 pounds max) has a lump
on her back/bum right at the base of her tail, to the left side and is about the size of a half dollar. It protrudes about a 1/2 inch if not more. It is soft, not hard and not causing her any pain. It disappears into her pelvis when pressing on it and will return in a very short amount of time (30 seconds - 1 minute or so). It seems to bulge farther when barking.