Ive also had cancerous prostatectomy, and0n the post operation notes from physio, we were instructed to exercise the bladder by taking in plenty of water to get bladder used to full volumes and get used to retention. Emptying bladder before it is full makes it lazy to learn to retain. Too little fluids makes urine concentrated and acidic, irritating bladder wall giving urgency to urinate more in the night. I also got up initially four times at nights first weeks but now its almost normal in forth week. Be blessed and receive healing.