Its very likely that your dad is correct. The spinal fusion (I'm sorry for your injury) creates lots of inflammation in the area. The Low Back (Lumbar - Sacrum) part of your spinal cord controls the nerves of your bladder, bowels, and other lower organ functions. Pressure on the spinal cord can will result in a less than optimal nerve control over the muscles that control your bladder. As your body heals, it is very likely that you will see a reduction or the symptoms will resolve. I would suggest taking an Omega - 3 as a dietary supplement to help reduce the level of inflammation in your body. Again this goes without know what caused the initial injury and result of the surgery.