Well, I took her to the vet. They said that normally after 2 weeks the knot from a rabies vaccination would go away, but it did feel like one of those reactions to them. So, she told me to keep an eye on it and if it got bigger or didn't go away in a month or so to immediately bring her back and they would cut it out and send it off for examination.
She's up to 12.5 lbs now, so they said she seems to be perfectly healthy and that this reaction is just taking a bit longer to go away for her.