Diverticulitis occurs when stool fills a diverticulum. The stool obstructs the opening between the diverticulum and the inside of the intestine. Despite the obstruction, the lining of the diverticulum continues to produce mucus. In addition, the bacteria that is normally inside the intestine start to overgrow inside the diverticulum. The mucus and bacteria irritate the diverticulum and cause it to swell.

The swelling and irritation to the diverticulum can cause pain and bleeding. In severe cases, diverticulitis can cause the diverticulum to burst, spilling stool outside the intestine and causing severe pain.