It definitely sounds like a puncture wound from a scratch. Mimi has had a couple in the few months, inflicted by one of our other cats.
Luckily hers have all healed without vet's visits, and she's been completely healthy otherwise, since money is incredibly tight right now. When I first discovered her bump I got so completely anxious, worrying that it cancer or something, which didn't quite fit because the bump became huge without my noticing it was there. After a while I managed to find the scab of where it started, and it calmed me slightly, so I watched her carefully, and the bump went down and things never went downhill... *fewf* Anyways, hope it turns out to be just that, and not something more serious! (Erik sends head licks)