Probably a combo of the wrist and a much more aggressive riding position. I'd wager you're putting your weight on your wrists, instead of gripping the tank with your legs and using your torso and back, and it is causing fatigue. Since the wrist was broken, it is probably causing pain faster than it normally would. (I know when I started riding, my wrists would get tired and start hurting after a couple hours.)
Try shifting your weight off of your wrists and see if that helps.
If not, maybe PM OfCounsel. He busted up his wrist pretty badly a couple years ago. Not sure if it was that exact bone, but he looked like an anatomy experiment for a while with all the pins and crap coming out of his wrist. He could probably offer some insight/tips/empathy.