Agreeing with the others that it's likely a vaccine reaction or a harmless fatty tumor (lipoma).
You might want to give your Canadian vet a call and find out what vaccine was given in that spot. I think most vets usually give different vaccines at different sites so if there is a reaction they know which one caused it. I know my vet group does that. I *think* most U.S. and Canadian vets follow the same protocol for which vaccine is given where (i.e., rabies is generally given in the right rear leg, etc.) If your Canadian vet gave them all in the same location . . I'd be looking for another vet.
