An anal fissure is an unnatural crack or tear in the anus skin. As a fissure, these tiny tears may show as bright red rectal bleeding and cause severe periodic pain after defecation. The tear usually extends from the anal opening and located posteriorly in the midline. This location is probably because of the relatively unsupported nature of the anal wall in that location.
Eh, that's not nearly as bad as the time one of my roommates bought single-ply toilet paper. I suffered slight discomfort on and off for like, two or three days.
Note to self: Probably shouldn't have done a Google Image Search of "anal fissure."