So maybe you have a dirty ass or or bad gear, or a combination of both. BUT I doubt it. It can happen more easily than you think.
Personally, as a registered nurse who worked trauma for 8 years(psych now)...I've seen MANY an abcess, caused by a variety of reasons. You can relax if you think it's your gear...I mean...it could be. But I doubt, your gear is bad or that you should toss it, per se...let me explain.
Even a perfectly sterile site, technique, procedure, injection, etc., can do/cause what happened with you.
The human body is very resilient to infection. You may have hypothetically speaking, been injecting bad gear the entire time you used it. It depositing in a very vascular area allows it to dissipate quickly enough for no inoculation to ever occur.. any that does or did, gets wiped out by our immune system quickly...luckily.
Moral of the story, don't take this for granted. Try to be as clean as possible, obviously. But understand, it could happen with perfectly sterile site. Sorry if you had to have it drained by a one Dr. Stepaniak in Houston, TX. He gets very aggressive with the forceps when breaking up the innoculation...he made a 320lb defensive lineman for the Texans cry like a little baby girl.