Diverticulitis: This is a possible diagnosis given the left lower quadrant abdominal pain and tenderness. Although diverticulitis often presents with fever and changes in bowel habits, the absence of these symptoms does not rule out the diagnosis, especially if it's a mild or early case.

Irritable Bowel Syndrome (IBS): The fluctuation between diarrhea and constipation, along with abdominal pain and changes in appetite, could suggest IBS. However, the weight loss and localized tenderness might not be fully explained by IBS alone.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis could explain the weight loss, changes in bowel habits, and abdominal pain. The absence of blood in stools and a more gradual onset might lean more towards Crohn's disease.
Gastroesophageal Reflux Disease (GERD) or Peptic Ulcer Disease: Although these conditions typically present with upper gastrointestinal symptoms, atypical presentations can occur, especially if the pain radiates or is referred to the lower abdomen.

Colorectal Cancer: Given the weight loss, changes in bowel habits, and abdominal pain, colorectal cancer must be considered, especially in the context of unexplained weight loss. Early detection is crucial, and this diagnosis should not be missed.
Appendicitis: Although the pain is on the left side, which is atypical for appendicitis, the condition can present variably, especially if the appendix is retrocecal. The absence of peritonism and fever does not rule out appendicitis.
Intestinal Obstruction: This could explain the fluctuating bowel habits and abdominal pain. However, the absence of more severe symptoms like severe vomiting or absolute constipation makes this less likely.

Intussusception: A condition where a part of the intestine telescopes into another, which can cause abdominal pain, changes in bowel habits, and potentially weight loss if chronic. It's rare in adults without a lead point like a tumor.
Mesenteric Ischemia: This condition involves insufficient blood flow to the intestines and can present with abdominal pain out of proportion to physical findings, weight loss, and changes in bowel habits. It's a rare but serious condition.
Eosinophilic Gastroenteritis: A rare condition characterized by eosinophilic infiltration of the gastrointestinal tract, which can cause a wide range of symptoms including abdominal pain, changes in bowel habits, and weight loss.