Nephrolithiasis (Kidney Stones): Given the patient's history of kidney stones and the presentation of bilateral low back/flank pain, this is the most likely diagnosis. The pain associated with kidney stones can be severe and is often described as crampy, and the history of kidney stones increases the likelihood of this diagnosis.

Pyelonephritis: An infection of the kidney, which can cause flank pain, fever, and other urinary symptoms. The patient's history of kidney stones increases the risk of developing pyelonephritis.
Musculoskeletal Strain: Given the location and description of the pain, a musculoskeletal strain could be a possible diagnosis, especially if the patient has recently engaged in heavy lifting or strenuous activity.
Degenerative Disc Disease or Lumbar Strain: These conditions can cause low back pain that may radiate to the flanks, although they are less likely to cause bilateral flank pain.

Abdominal Aortic Aneurysm: Although less common, an abdominal aortic aneurysm can present with severe back or flank pain and is a medical emergency. The risk factors for this condition include age, smoking, and hypertension.
Sepsis: If the patient has a severe infection, such as pyelonephritis or another source of infection, sepsis could be a life-threatening complication that needs to be considered.
Renal Infarction: A blockage of the renal arteries can cause severe flank pain and is a medical emergency.

Papillary Necrosis: A condition where part of the kidney tissue dies due to a lack of blood supply, often associated with analgesic abuse, diabetes, or sickle cell disease.
Retroperitoneal Fibrosis: A rare condition characterized by the proliferation of fibrous tissue in the retroperitoneum, which can cause back and flank pain due to the encasement of structures like the ureters.
Spinal Infection (e.g., Osteomyelitis, Discitis): Infections of the spine can cause severe back pain and are more common in individuals with certain risk factors, such as intravenous drug use or compromised immune systems.