sterile abscess is caused when you miss the muscle when you inject and the oil clumps up in a ball. you will either have to have it drained out which is difficult or you will have to just leave it alone and let it shrink up on its own over time.. it can take a while. it doesn't hurt or anything but it will be a pea sized lump
infected abcess you will likely need anti-biotics, that is gonna be red/swollen.. possibly hot to the touch. that obviously is caused by an infection which can happen for many reasons.