If it's the "back" system and the blood is black, then the problem is an upper GI bleed, and needs to be attended immediately. Like now.
If the blood is red, or frank, it's lower GI, and could be a number of things - IBS, ulcerative colitis, diverticulitis, anal fissure, anal fistula or thrombosed hemorrhoids. These all require medical attention, soon.
Bt