Think you may be tensing up your shoulders while shooting plus dropping your head to line up the sights. I know this is going to be a PITA, but see if raising the pistol up to you line of sight instead of dropping you head, helps.
Rifle shooting is a bit tougher to mend, I hurt my shoulder/neck in '03 and I still get stiff shooting a long gun.
Hmmm ... OK, you are right about me pulling my shoulders up and dropping my head. Just habit from shooting on the move. I'll give that a try. Thanks