Hi David,
 
The mass you felt may be a tumor, and I would recommend taking Jinx to the
vet ASAP for a diagnosis.  Last fall, my ferret Alfalfa was at the vet for
his yearly vaccinations when the vet discovered a similar mass.  I hadn't
noticed it before, and Al seemed to feel fine.  The vet removed it and told
me that Al would probably have died within a month or two, because the
mass, which was a tumor, was about to rupture and would have caused him to
bleed to death internally.  Luckily, my wonderful vet discovered it first,
and Al is fine now.
 
Best wishes for you and Jinx,
Nancy and critters
[Posted in FML issue 3509]