Just a thought - you said it wasn't there, and then it was.  It probably
doesn't look like a bite or cut or puncture... is it somewhat raised?  Is
it small, like an ant bite on a human?
 
If so, it reminds me of some strange sores my late Guiniviere had.  They
were under the fur on her neck.  They were raised, sort of like a small
ant bite, but open; although bloody looking, they did not run.
 
The vet diagnosed mast cell tumors.
 
It was rather inexpensive to have them removed.  It seems though that they
were a result of other health factors, not the cause.  Guin also suffered
from a melanoma and pancreatic cysts, and -perhaps- they indicated metastasy
(sp?) elsewhere.
 
a
[Posted in FML issue 2244]