Nephrolithiasis (Kidney Stones): Given the presentation of flank pain, this is a common cause that can be easily confirmed with imaging. The patient's age and symptoms align well with this diagnosis, despite the history of lymphoma.

Pyelonephritis or Urinary Tract Infection (UTI): Although the UA is negative, it does not entirely rule out an infection, especially in an elderly patient where symptoms and test results can be atypical.
Musculoskeletal Pain: Elderly patients can experience musculoskeletal pain due to various reasons such as degenerative changes, which could be exacerbated by activity or position, mimicking flank pain.
Lymphoma Involvement: Given the patient's history of ALK negative lymphoma, there's a possibility that the flank pain could be related to lymphoma involvement in the kidneys or surrounding tissues, although this would be less common.

Renal Cell Carcinoma: A new onset of flank pain in an elderly patient warrants consideration of renal cell carcinoma, especially if there are any abnormalities on imaging.
Aortic Aneurysm or Dissection: Although less common, an aortic aneurysm or dissection can present with flank pain and is a medical emergency that requires immediate attention.
Pulmonary Embolism: While the presentation might not be typical, pulmonary embolism can sometimes cause referred pain that might be perceived as flank pain, especially if there is involvement of the upper lobes.

Retroperitoneal Fibrosis: A rare condition that can cause flank pain due to the formation of fibrotic tissue in the retroperitoneum, which can encase and obstruct structures such as the ureters.
Amyloidosis: Involvement of the kidneys by amyloid deposits can lead to various symptoms including flank pain, although this is relatively rare and usually associated with systemic symptoms.
Sickle Cell Disease Crisis: If the patient has sickle cell disease (less likely given the age and no mention of it), a crisis could cause flank pain due to sickling in the kidneys.