Well, the lump is almost gone-- just a little scab now.
I talked with our vet at work and she thought it sounded strange, but said not to worry too much since it has gotten better. She did say that if it seems to come back, get him in to his vet right away.
Could have been a bite, staph infection that went into the skin, or a foreign body that was working its way out. Since it wasn't pus, she didn't think he needed antibiotics.