They said it's too small to take a test sample. And they didn't mention either of those. They told me to monitor it for now, and if it gets bigger take her back ASAP. He said that it could just be irritation from the shot - because it's in the same area where they usually do the rabies shot.
Yea that's pretty common with cats to get irritation from injection sites. They're also pretty sensitive to benadryl but some vets will say to try it. If it gets bigger, then yea I'd call them but it's possible it could go down in the next couple of weeks. If your cat isn't acting ill or off, I wouldn't worry too much for the moment.