To the ANON poster; My Chumley (2 yo) recently developed what sounds like
the same thing.  It is BB sized and showed up overnight.  I took him to
the vet and it turned out to be a harmless cyst.  I would suggest you take
Stanley to your vet and have it checked out.  They need to examine it's
firmness, and make sure it's not interfering with muscle, tendon, or
ligament function.  This is done easily, but I will suggest you have some
Ferretone or other munchie because the vet will most likely want to shave
the area for a good look.
 
  Good luck!
  SP
[Posted in FML issue 2238]