At the risk of being the 100th poster about the bump on the neck of the
ferret, let me say that it is very very likely due to the rabies
vaccination.  We have had several react the same way, and the first time it
happened, I reacted the same way Claudia did.  Our vet, Dr. Dutton, told me
not to worry, it would go away.  It did.
 
I have seen it since in others, usually a short time after the rabies
vaccination.
 
Dick B.
[Posted in FML issue 1838]