Splenomegaly or Splenic Infarction: The presence of a lump in the left upper quadrant, along with intermittent pain, suggests an issue related to the spleen. Splenomegaly (enlarged spleen) or splenic infarction could cause these symptoms. The stable vital signs and lack of nausea or tachycardia do not immediately suggest an acute abdomen, but the spleen's location corresponds with the area of pain.

Gastrointestinal Issues (e.g., Gastritis, Ulcer): Although the pain is localized to the left upper quadrant, gastrointestinal issues such as gastritis or an ulcer could refer pain to this area. The absence of nausea and the specific location of pain make this less likely but still a consideration.
Constipation or Bowel Obstruction: While less common for constipation or bowel obstruction to present with pain solely in the left upper quadrant, it's possible, especially if there's a specific segment of the bowel involved.
Musculoskeletal Pain: Sometimes, musculoskeletal issues can mimic abdominal pain. Given the intermittent nature of the pain and the lack of systemic symptoms, this could be a consideration.

Ruptured Spleen or Splenic Aneurysm: Although the patient is stable, a ruptured spleen or splenic aneurysm could present with similar symptoms and would be catastrophic if missed. The presence of a lump and pain in the left upper quadrant necessitates careful consideration of these diagnoses.
Pancreatitis: While pancreatitis typically presents with more systemic symptoms like nausea and vomiting, it can sometimes have a more subtle presentation, especially if it's chronic. The location of pain could be consistent with pancreatitis, and missing this diagnosis could have significant consequences.
Abdominal Aortic Aneurysm (AAA): Although AAA typically presents with back or mid-abdominal pain, it's conceivable that an aneurysm could cause referred pain to the left upper quadrant. Given the potential lethality of a ruptured AAA, it's crucial not to miss this diagnosis.

Left Upper Quadrant Mass (e.g., Tumor): A mass in the left upper quadrant could cause the described symptoms. While less common, tumors (either primary or metastatic) could present in this manner.
Inflammatory or Infectious Processes (e.g., Abscess): An abscess or other inflammatory process in the spleen or adjacent tissues could cause localized pain and a palpable mass. These conditions are less common but should be considered, especially if the patient has risk factors for such conditions.