about your shoulder:
while not on your bike, pull your shoulders way back. WAY back and stick out your chest like you are hot stuff (which you are).
try to touch your shoulderblades together.
do that a lot.

sometimes that pain is because your shoulders (like almost everyone who sits in front of the computer) are rotated forward and they ARE pinching a nerve.

why it's happening on your bike instead of at home i don't know, but you can also focus on softening your shoulders while you are riding.
good luck.