Peptic Ulcer Disease: This condition is a common cause of gnawing or burning pain in the upper abdomen, which can sometimes be localized to the left upper quadrant. The pain is often accompanied by nausea and can be exacerbated by eating or relieved by antacids. Chills might not be a primary symptom but could occur if there's an associated infection or perforation.

Gastroesophageal Reflux Disease (GERD): While GERD typically causes heartburn and regurgitation, some patients may experience upper abdominal pain, including in the LUQ, along with nausea. Chills are less common but could be present in severe cases or if there's an esophageal infection.
Pancreatitis: Pancreatitis can cause severe, persistent upper abdominal pain that radiates to the back, nausea, vomiting, and sometimes chills if there's an infectious component. The pain can be diffuse but might be perceived in the LUQ.
Cholecystitis or Cholangitis: Although these conditions typically cause right upper quadrant pain, some patients may experience pain in the LUQ, especially if there's a stone in the common bile duct causing referred pain. Nausea, vomiting, and chills are common due to the inflammatory or infectious nature of these conditions.

Splenic Infarction or Rupture: These are medical emergencies that can present with sudden onset of severe LUQ pain, nausea, and potentially chills if there's an infectious cause. Given the potential for rapid deterioration, these conditions must be considered and ruled out promptly.
Pneumonia (especially left lower lobe): Pneumonia can cause referred pain to the abdomen, especially if the diaphragm is involved. Chills are a common symptom, along with fever and cough. The pain might be perceived in the LUQ if the pneumonia affects the lower lobe of the left lung.
Myocardial Infarction: Although typically presenting with chest pain, myocardial infarction can sometimes cause referred pain to the abdomen, including the LUQ. Nausea and chills might be present, especially if there's associated heart failure or arrhythmia.

Splenic Abscess: A rare condition that can cause LUQ pain, fever, chills, and nausea. It's often associated with immunocompromised states or after splenic trauma.
Left Upper Quadrant Mass or Tumor: Various tumors (e.g., gastric, pancreatic, or splenic) can cause LUQ pain, nausea, and potentially chills if there's tumor necrosis or infection. These are less common causes of the described symptoms but should be considered in the differential diagnosis, especially if other causes are ruled out.