What Causes Abdominal Pain And Blood In Stool?

Hi, I occasionally suffer from minor abdominal discomfort/pain. I don t have the best diet, I m a bit over weight, and until 4 days ago I regularly smoked cigarettes. After passing a bowel movement today, I noticed a patch of blood on my tissue paper after wiping that was roughly the size of a cookie, maybe a bit bigger. However, I didn t see much if any blood in my stool but did notice tiny traces of it on the outside. This has never happened to me before and now I am experience subtle abdominal discomfort. I m really nervous, I m 21 years old and had a few drinks tonight. What is wrong with me?