I'm an experienced guitarist/pianist and am having a bit of a problem with my left hand. When I'm playing guitar I seem to suddenly get a pain between my index finger and thumb fairly deep, almost as if I'm a beginner again. It's playing havoc with my ability to make it through an entire song without pain. Also my hand has started to outright refuse to move in certain directions even after a warm up.
Barre chords are agony to attempt almost immediately with a thick-necked guitar and even any octave stretches on piano are painful. I'm hoping that I've just pulled a muscle but it's been around for the last 3 months or so. I've never really liked the thought of hand trainers (to me it always sounded like the old dipping your fingers in vinegar to get them to harden up) but if this will allow me to play again I'm happy to try it.
Any recommendations for relaxing/training that muscle?
Edit: As a side note, my hands lock up occasionally and I need to heat them up in order to be able to get all feeling and movement back.