Bump after pen shot

Question
My rescue dog, Dez, had surgery on Thursday to remove a mass on the underside of her tail.  To help insure that she didn't get an infection the vet gave her a subQ penicillin shot.  The site where the shot was given has developed a hard bump about the size of two marbles.  It doesn't seem to bother her at all, but should I be concerned?

Answer
It could be a couple of things, one there could've been a small amount of blood leakage back toward the syringe during the vaccination. It would've gathered under the skin and will go down on it's own as it is resorbed. Or 2, it could be a mild allergic reaction to the pen. shot. Make sure you mention it to the vet next time you're in (presumably for a recheck of her wound where the mass was removed) and ask them to note it on her record. Next time they might want to try a different antibiotic. Usually allergic reactions increase in severity with each exposure. You don't' want your girl to have an anaphylactic reaction which could be serious. there are other, better antibiotics.
Diane