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.

Here are some. .. Clinically, I agree with my colleague stating musculoskeletal origin as a culprit for pain as described. For a 91-year-old man, bring the detailed info on his voiding history and pain near left hip & lower back stressing their quality, quantity, and sequence of events over time to Doc for analysis, physical exam, & tests such as residual urine, urinalysis, renal function, etc. So to define cause.
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.