You accidentally cut your butt deep inside! Wow, I don't think you were attempting to shave the insides of your intestines! Go to the emergency room, it will be embarrassing, butt (pun intended) you need medical attention. Given what exits that area you could be getting a lot of crap in your blood stream!
Get into a warm tub or take a sitz bath to decrease the pain and relax the muscles. Drin more fluids to soften your bowel movement and eat foods high in fiber such as fruits, vegetables, and whole grains. If the pain worsens, see a doctor to ascertain the extent of the cut.
There's really nothing you can do besides wait for it to heal. Eating lots of fruits (vitamin A) will speed up the healing process) and I know this will burn but apart your cheeks and put an ice pack in. It will sooth the cut better than heat.