As with any other physical activity, STOP AS SOON AS IT BECOMES PAINFUL!!!!!
The fact that your wrist is bent shouldn't be causing you any pain, but you might have a pinched nerve which is being exasperated by the way you are standing and the weight of the guitar strap pushing down on said nerve. (Very common)
If this happens every time you play standing up, you might need to simply stretch your spine out before playing, I know that many guitar players hunch when playing standing up.
If it feels like the pain is muscular in origin, you should stretch your wrists out slowly prior to playing, and run em under warm water for a few minutes. This will loosen the hand muscles and warm em up.
You might want to start off playing standing up for shorter periods of time and build up, too.