I don't know what caused it, but last year I had one after a sugery on my Rt leg, high fever, swelling in the sugery area, vomiting, my foot turned a dark red were the bad blood started to settle. The doc had to open the wound and let it drain and then they gave me huge doses of IV antibiotics, I was in the hospital a week and felt like shit for a few months, very nasty.
you run the riskof getting an infection anytime you poke yourself. even if your gear is clean a freak happenstance event can occur and you end up with an infection. always swab the area with an alcohol prep and use a new needle and syringe. never ever re-use or share with another person. if your gear is dirty or unfiltered then you run even higher risks. always use gear thats been filtered because all the ba in the world won't keep you safe from small particles that are injected into your muscle.
the infection can range from a mild cellulitis to a full blown intramuscular abscess. that would be absolutely horrible to deal with and CANNOT be put on the back burner. signs of infection are pain, swelling, redness that will begin spreading from the foci of infection and you may or may not see pus or a discharge. you may also have fever. note that you must distinguish this from the pain, swelling, and redness that you will sometimes get when you inject high dose test blends and test prop. these should go away after about 2 days. everybody knows what it feels like to use a ml of bl mega test or remembers that first time they shot 3ml's of prop.
you may be in store for a course of iv antibiotics in the hospital and some pills to take once they let you out. if you catch it quick though, an im shot of a gram of rocephin along with a course of cephalexin may get you all fixed up. once there is a big fluctulant mass (soft in the middle meaning there is pus), you are gonna have to get it opened and drained. while its still indurated (red and hard with no head), use warm compresses to help it resolve/come to a head.