My arm hurts every time i throw down to 2nd? why?

Like BQS_UNIVERSAL points out , stretch , stretch and stretch again during warm ups, I would make that mistake myself as an adult catching at times and the throw to 2nd would remind me. The other thing to be careful for , make sure your following thru with your arm after the throw , don`t be stopping it short after letting go , your throwing hard and fast from the crouch , let it go , and let your body flail if it wants to after with the inertia created. In time yu`ll get it down either from the crouch and falling forward or stepping up and forward.