Pooping pure blood, help?

The past month or so I've been passing blood in my stool and I just thought the normal, it's a hemorrhoid. But just now I used to bathroom and passed just PURE blood. Little to no poop at all. I'm a little worried. Immediately after my stomach began hurting. I did eat a lot today, but that has nothing to do with the blood. I consumed maybe 1600-1800 calories. I am 18 and know there's not much of a possibility of colon cancer, because I'm so young, but could it be a bowel disease? I think I need to see a doctor but how urgent is it? Please help.