91 year old male with pain on left side about hip level but towards the back. no blood in urine. could this be a kidney stone on coumadin and flomax (tamsulosin). he has a huge protate and does not hydrate enough.

More : More likely, this is musculoskeletal pain from the back/hip, not kidney. A renal ultrasound would be reasonable to exclude swelling of the kidney, followed by imaging of the hip and lumbar spine.

Urinary retention...: Severe urinary retention can lead to pain associated to a uti, dilated renal system (hydroureter, hydronephrosis), or else a kidney infection (pyelonephritis) especially if a history of an enlarged prostate exists and even if he is prescribed flomax (tamsulosin). A visit to the doctor is definitely needed since the elderly may not show typical symptoms of an adult with a complex urinary tract infection.