Musculoskeletal strain: Given the absence of systemic symptoms (such as fever) and negative urinalysis and pregnancy test, musculoskeletal strain is a common and plausible cause for non-radiating flank pain. This could be due to overuse, injury, or poor posture.

Constipation: Constipation can cause abdominal and flank discomfort due to the proximity of the colon to the flank area. The pain is often non-radiating and can be associated with bowel movements.
Gastrointestinal issues (e.g., gastroesophageal reflux disease (GERD), inflammatory bowel disease): Although less common, gastrointestinal issues can sometimes present with flank pain, especially if there is involvement of the stomach or the intestines near the flank area.
Herniated disk or spinal stenosis: These conditions can cause flank pain due to nerve root irritation. The pain might not always radiate down the leg and can be confined to the flank area.

Abdominal aortic aneurysm (AAA): Although less likely given the patient's afebrile status and lack of radiating pain, AAA is a critical diagnosis that must be considered, especially in older patients or those with risk factors such as smoking or hypertension. A missed diagnosis can be fatal.
Pulmonary embolism: While typically presenting with respiratory symptoms, in rare cases, a pulmonary embolism can cause flank pain if the embolism affects the upper parts of the lung. This diagnosis is critical and potentially life-threatening.
Splenic infarct or rupture: These conditions can present with sudden onset of left flank pain. Given the potential for severe consequences, including hemorrhage, they must be considered, even though they might be less likely without a history of trauma or underlying splenic disease.

Pancreatic cancer or pancreatitis: These conditions can cause flank pain, especially if the pancreatic tail is involved. However, they are less common and usually present with other symptoms such as weight loss, jaundice, or abdominal pain.
Renal cell carcinoma: Although a possible cause of flank pain, renal cell carcinoma is rare and typically presents with other symptoms such as hematuria, which is not indicated by the negative urinalysis in this case.
Lymphoma: Can cause flank pain due to lymph node enlargement or involvement of nearby structures, but it is a rare diagnosis and usually accompanied by systemic symptoms or other signs of malignancy.