What is this horrible rectal pain? i have really thin stools.

Please see your Dr.: There are a number of possible causes for rectal pain; the only way to determine the diagnosis is to see your doctor. Your doctor will need to examine the outside of the anus and will likely need to do a rectal exam as well. If your doctor does not find a cause, seeing a gastroenterologist (gi specialist) would be the next step.

Fissure, maybe...: You may have a fissure- an acute crack or tear in the anal canal that is quite painful. Spasm of pelvic floor muscles and nonrelaxation of the sphincter can make it difficult to move bowels. Drink plenty of water, try a stool sotener, but above all, see your doctor asap for diagnosis in person, and to rule out more concerning issues that may be present.