A bum bubble generally occurs when you're about to fart; as the gass being released is about to exit your anus, a bubble is created. When the bubble remains in your anus, and fails to be pushed out, it is known as a bum bubble; however, it can sometimes be mistaken for a sweaty anus.
stephen - whats up josh st. G, you look kinda funny
josh st. g - ya i've had this bum bubble stuck in my anus for hours and it's starting to really chafe, i even saw a little bit of blood squirt out, i'm hoping when i shit it'll pop
stephen - those sick bum bubbles