Boils are similar in appearance to abscesses, but are more likely to be found at the site of the entry or exit points of the piercing. They can occur anywhere and are highly unlikely to develop orally, but are commonly found on nostril and nipple piercings. They also tend to develop when a piercing has been snagged and torn the skin.
Bacteria, usually on the skin surface, get trapped inside the wound by skin tissue sealing it in. The trapped pus is often close to the surface of the skin and some people pop them as they would a spot. It is better, however, to treat them with a hot compress a couple of times a day. Allow the boil to bleed but make sure that you clean and dry the site so no further germs or bacteria can invade.