Diverticulitis: This condition is a common cause of left lower abdominal pain, especially in older adults. The pain is typically localized to the left lower quadrant and may be accompanied by changes in bowel habits, fever, and nausea.

Irritable Bowel Syndrome (IBS): IBS can cause chronic or recurrent abdominal pain, often localized to the left lower quadrant, accompanied by changes in bowel habits such as constipation or diarrhea.
Constipation: Severe constipation can cause significant abdominal pain, particularly if there is a large fecal impaction, which might be felt more prominently in the left lower abdomen due to the location of the sigmoid colon.
Ovarian Cyst or Torsion: In females, ovarian cysts can cause intermittent or constant pain, usually on one side, and can be accompanied by other symptoms like menstrual irregularities or nausea.
Pelvic Inflammatory Disease (PID): PID can cause lower abdominal pain, often bilateral but can be unilateral, and is typically accompanied by other symptoms such as fever, vaginal discharge, and pain during sexual intercourse.

Ectopic Pregnancy: Although less common, an ectopic pregnancy is a life-threatening condition that must be considered, especially in women of childbearing age presenting with abdominal pain and a missed period or positive pregnancy test.
Appendicitis (if pain has shifted): While appendicitis typically presents with right lower quadrant pain, in some cases (especially if the appendix is retrocecal), the pain might initially be more diffuse or on the left side before localizing to the right lower quadrant.
Intestinal Obstruction: This is a serious condition that can cause severe abdominal pain, vomiting, and constipation. The pain can be diffuse or localized, depending on the site of obstruction.
Ruptured Abdominal Aortic Aneurysm: This is a medical emergency that can present with severe abdominal or back pain and is more common in older adults with risk factors for vascular disease.

Inflammatory Bowel Disease (Crohn's Disease or Ulcerative Colitis): These conditions can cause chronic abdominal pain, diarrhea, weight loss, and other systemic symptoms. While not as common as other causes, they are important to consider in the differential diagnosis.
Endometriosis: This condition can cause chronic pelvic pain, dysmenorrhea, and dyspareunia, and is more common in women of reproductive age.
Colonic Cancer: Although rare in younger populations, colonic cancer can cause abdominal pain, changes in bowel habits, and weight loss, and should be considered, especially in individuals with risk factors or familial predisposition.