Hi rpharley:
This is not really uncommon to happen after vaccinations and is referred to as a vaccination sarcoma. Normally it resolves itself after a few to several weeks so do keep an eye on it. If it gets larger than a grape size, it should be checked out by your veterinarian. You can read all about these below:
http://www.avma.org/vafstf/ownbroch.asp
Good luck to you and you kitty :)
breeder/healthcare specialist
15+ years animal health care Hobby breeder of Olde English Mastiffs and owned by an English Bulldog