Irritable Bowel Syndrome (IBS): This is the most likely diagnosis due to the combination of lower abdominal pain, constipation, and diarrhea, which are hallmark symptoms of IBS. The condition is common and often presents with a mix of these gastrointestinal symptoms without an identifiable cause.

Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis can cause lower abdominal pain, changes in bowel habits (including both constipation and diarrhea), and other symptoms. The presence of blood in stool, weight loss, or fever might suggest IBD over IBS.
Diverticulitis: Inflammation of the diverticula in the colon can cause lower abdominal pain, changes in bowel habits, and potentially fever. It's more common in older adults.
Gastroesophageal Reflux Disease (GERD): Although primarily associated with upper GI symptoms, GERD can sometimes cause lower abdominal discomfort and changes in bowel habits due to the reflux of stomach contents.

Appendicitis: Although appendicitis typically presents with right lower quadrant pain, atypical presentations can occur, especially in children or the elderly. It's crucial to consider appendicitis due to its potential for severe complications if not promptly treated.
Ovarian Torsion: A gynecological emergency that can cause severe lower abdominal pain, which might be accompanied by nausea, vomiting, and changes in bowel habits due to the proximity of the ovaries to the intestines.
Intestinal Obstruction: This condition can cause severe abdominal pain, constipation (or obstipation), and potentially diarrhea, depending on the level and completeness of the obstruction. Vomiting and abdominal distension are also common.
Ectopic Pregnancy: Should be considered in women of childbearing age presenting with lower abdominal pain and any change in menstrual pattern or suspicion of pregnancy. It's a medical emergency.

Celiac Disease: An autoimmune reaction to gluten that can cause abdominal pain, diarrhea, and constipation, among other symptoms. It's less common but should be considered, especially with a family history or other autoimmune conditions.
Colonic Cancer: Although rare in younger populations, it can cause changes in bowel habits, abdominal pain, and potentially weight loss. Risk factors include family history, age, and certain genetic syndromes.
Neuroendocrine Tumors: Rare tumors that can produce various peptides leading to distinct syndromes, some of which include abdominal pain and changes in bowel habits.
Malrotation with Volvulus: A rare condition where the intestine twists around itself, cutting off its blood supply. It can cause severe abdominal pain, vomiting, and changes in bowel habits, and is a surgical emergency.