Blood in the Stool
One of the most common symptoms of colitis is diarrhea [2] with blood or pus in it. After you are finished relieving yourself, you can see this blood or pus in the toilet. However, despite its presence in your stool, it is also not unusual for the blood [3] to not be visibly seen.
The inflammatory process contributes to diarrhea, often causing the colon to constantly empty itself out. As the cells that line the inside of your colon die and grow, open ulcers begin to form (which also lead to pain). This is what leads to the development of pus and mucus. The intestinal wall may also start to bleed. What causes the bloody stool in colitis is pus development and bleeding from the colon wall.
This internal bleeding from the ulcerative effect on the colon wall usually leads to the washing away of iron from the body as it is lost in the blood. This can cause iron deficiency [4].