Hello, although taking warfarin is a potential cause of urinary tract bleeding, back pain and bleeding also raises concerns for renal problems like stones in the kidney. A radiological assessment of the urinary tract would help a lot to determine the cause. You should also check your INR (or prothrombin time). If it is higher than 4.00 then this is a good reason even for spontaneous bleeding. In that case (and depending on the INR) you should consult your doctor for appropriate treatment and dosage modification. If the INR is too high, more aggressive measure may have to be used like transfusions of fresh frozen plasma or vitamin K administration. I hope you find my comments helpful! You can contact me again, if you d like any clarification or further information. Kind Regards!