Diverticulitis: This condition is characterized by inflammation of the diverticula in the colon, which can cause severe lower left abdominal pain, fever, and changes in bowel habits. It is a common cause of lower left abdominal pain, especially in older adults.

Constipation: Severe constipation can cause significant abdominal pain, including in the lower left quadrant, due to the accumulation of feces in the sigmoid colon.
Irritable Bowel Syndrome (IBS): IBS can cause recurring abdominal pain, often in the lower left quadrant, associated with changes in bowel habits, such as diarrhea or constipation.
Ovarian Cyst or Torsion: In females, an ovarian cyst can cause lower abdominal pain if it ruptures or becomes twisted, cutting off its blood supply.
Kidney Stone: A stone in the left ureter can cause severe pain in the lower left abdomen, radiating to the groin, and is often accompanied by nausea, vomiting, and urinary frequency.

Appendicitis: Although appendicitis typically presents with right lower quadrant pain, in some cases (especially if the appendix is retrocecal), the pain can be felt in the lower left abdomen. Missing this diagnosis can lead to perforation and peritonitis.
Ectopic Pregnancy: In pregnant women, an ectopic pregnancy can cause severe lower abdominal pain, which is a medical emergency requiring immediate attention.
Bowel Obstruction: An obstruction in the sigmoid colon can cause severe lower left abdominal pain, vomiting, and constipation, and requires prompt surgical intervention.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can cause chronic inflammation and damage to the digestive tract, leading to severe abdominal pain, including in the lower left quadrant.

Intussusception: A condition where a part of the intestine slides into an adjacent part, which can cause severe abdominal pain, vomiting, and bloody stools.
Mesenteric Ischemia: Reduced blood flow to the intestines can cause severe abdominal pain, which may be out of proportion to the physical examination findings.
Endometriosis: In some cases, endometriosis can cause cyclic lower abdominal pain, especially if there are endometrial implants on the bowel or other pelvic structures.
Colonic Cancer: Although less common as an initial presentation, colonic cancer can cause abdominal pain, changes in bowel habits, and weight loss, and should be considered in the differential diagnosis, especially in older adults or those with risk factors.