Why does your anus hurt after diarrhea?

I'm a college freshman and I sometimes drink too many liquid (water, soda [no caf], milk), and I've had diarrhea a few times.

I be just wondering, why does your butt hurt after you acquire diarrhea, and sometimes bleed too? Too much toilet paper? Undigested bits of food scrape? Leftover gastric acid? Too much coming out at ones?

Just curious.

(P.S. Yahoo considers b*u*t*t a dirty word. But not anus.)
Answers:    becaue it is constantly dialated, and tierd of one open. horrid but true.