INTESTINAL FORTITUDE by Joshua Scribner is centered on someone who has spent his life being attacked, blacking out, and waking up to his assailants being killed, over and over. At the onset of each attack, he suffers severe abdominal pain, and when he wakes up, he finds he has crapped his pants.
As an adult, one of his attacks is finally caught on a security camera, and he discovers what has been happening after he blacks out. Scribner delivers one hell of an ending. I highly recommend this one.