Am 19 and I am passing stools with little bright red blood and sometimes with mucus for 3 months what is its cause and sometimes it also cause abdomin?

See doctor asap. Blood and mucus in stool is a sign of an intestinal infection, or some other disorder of the gastrointestinal tract. If the blood is red, it is in the colon, indicating colitis; if dark red or black, upper GI bleeding such as an ulcer. Your doctor will examine you, do some tests on your stool, blood tests, and do other diagnostic studies to make a diagnosis and treat you. Do this asap- be well!