Andy,

More than likely the fatigue is being caused by a combination of things. The most prominent causes of it are probably the pain and the pain killers.

Pain puts your body in distress and also will cause fatigue both physically and mentally. Opiate pain killers also cause fatigue, they depress the central nervous system, slowing the cardiovascular system making physical activity more difficult by lowering blood pressure and the amount of oxygen delivered to your system.

You've also just been through surgery so your body is in a hypermetabolic state (healing) so your body is burning through calories at the moment, add in the psychological trama (realized or not) and you have a recipe for fatigue brotha. Give yourself some time Superman you've been exposed to kryptonite.

Keep your chin up

Eric