I've had the same problem with my cougar. It's an ergonomic thing.
If the joystick and throttle aren't positioned at the correct height and horizontal position, then there will be strain on one side of the solder or the other, or even both.
My problem was only with the joystick. The height and horizontal, (forward or back) position of the throttle was fine.
I fixed the problem by setting the height of the joystick to the same height as the throttle. Both my joystick and throttle are mounted on adjustable shelves. I can slide the shelf out and close to my body, or in and farther away. I played with this position for the joystick until I found the spot that didn't cause any more shoulder pain.
It's been working fine for months now for me.