They do not necessarily mean that a person has diverticulosis. If an individual has these symptoms, he or she should see a health-care professional. Diverticulitis is a more serious condition and causes symptoms in most people with the condition that include: 1  pain in the abdomen, usually in the lower left side; 2  bleeding, bright red or maroon blood may appear in the stool, in the toilet (a symptom of rectal bleeding), or on the toilet paper.