from a doctor--it is a really strong antibiotic usually prescribed for gear infections (in some cases bros get it via IV b/c they wait to long and by the time they get the ER it's the only way). Abscesses are usually a bit worse than just warm, though. It may just be a reaction to the gear/ the BA/BB mix may be off.
Some bros who have had abscesses in the past are scared enough they keep their own supply of Keflex--I just saved mine from dental procedure.
this is assuming you have an abscess and it's not just typical gear reaction--I get golfballs from Primo that stay that way for a few days...after a while you know what is nothing to worry about and what is a problem (although I never had one yet--knock wood).