The most common and severe sign of diverticulitis is sudden pain on the lower left side of the abdomen. This pain can sometimes get worse over a few days. Other signs that you might have diverticulitis are: 1  abdominal pain and tenderness, usually on the lower left side. 2  nausea. 3  vomiting. 4  constipation. 5  fever. 6  gas or bloating. 7  diarrhea. 8  loss of appetite.