Gastroenteritis: This is the most likely diagnosis given the combination of left upper quadrant pain, changes in bowel movement, nausea, and anorexia. Gastroenteritis, often caused by viral or bacterial infections, can lead to these symptoms due to inflammation of the stomach and intestines.

Constipation: Changes in bowel movements could indicate constipation, which can cause left upper quadrant pain due to the proximity of the colon to this area. Nausea and anorexia can also be associated with constipation.
Gastritis: Inflammation of the stomach lining can cause upper abdominal pain, nausea, vomiting, and changes in appetite, fitting the described symptoms.
Irritable Bowel Syndrome (IBS): Although more chronic, IBS can present with abdominal pain, changes in bowel habits, and nausea, making it a consideration in this scenario.

Appendicitis: Although appendicitis typically presents with right lower quadrant pain, it can occasionally present atypically, especially if the appendix is retrocecal. Missing this diagnosis can lead to severe consequences, including perforation and peritonitis.
Diverticulitis: Inflammation of a diverticulum in the colon can cause severe abdominal pain, changes in bowel habits, and systemic symptoms like fever. It's more common on the left side and requires prompt treatment to avoid complications.
Intestinal Obstruction: This is a surgical emergency that can present with abdominal pain, nausea, vomiting, and changes in bowel habits. Early diagnosis is crucial to prevent severe complications.
Pancreatitis: Pancreatic inflammation can cause severe upper abdominal pain radiating to the back, nausea, vomiting, and can be associated with changes in bowel movements and appetite. It's a serious condition that requires immediate medical attention.

Splenic Infarction or Rupture: These conditions can cause left upper quadrant pain and might be associated with nausea and anorexia. They are less common but can be life-threatening and require immediate medical intervention.
Gastric Volvulus: A rare condition where the stomach twists abnormally, causing severe upper abdominal pain, nausea, and vomiting. It's a surgical emergency.
Left-Sided Inflammatory Bowel Disease: Conditions like Crohn's disease or ulcerative colitis can cause chronic inflammation of the bowel, leading to abdominal pain, changes in bowel habits, and systemic symptoms. They are less likely to present acutely but should be considered in the differential diagnosis.