It may not be an "allergic reaction"; necessarily but just the fluid from the injection. I used to work as a vet's assistant; and a LOT of dogs would have a tiny bump (now I'm just talking like, between the size of a pea or a dime) where the injection was given; since they are subcutaneous instead of given in the muscle. So the fluid/vaccine just goes right under the skin and forms a little bubble. Rarely will the dog seem to notice any problems from it, and it would be with ANY sub-q (just under the surface of the skin) vaccine, not just rabies. It happens frequently but seems that only those who have small dogs that get handled a lot notice LOL... so for us with chihuahuas we pick up on it right away; whereas a lab or golden with an equally tiny bump under all that fur would get unnoticed. Eventually it should just all absorb, but sometimes can take time. As long as the dog doesn't show any signs of a reaction; itching, vomiting, etc. I'd say you've got nothing to worry about!