Anybody have some advice for this? I've been playing for about 9 years now and I've NEVER been able to get rid of this pain in my shoulder when playing. It's greatly reduced my ability to play faster. Palm muting seems to be extra bad for it. I can actually feel the tension in my shoulder when playing. :/

Im beginning to wonder if its my technique. Is the only answer practice and try to consciously feel it and relax? I just can't seem to stop it :/
I had a simmilar problem when I started out. My teacher adviced me to slow down, and try picking using only the wrist movement while keeping all other muscles relaxed. When trying to play fast, try staying conscious on the shoulder, and follow the rule "less movement while picking is better".

Ive tried that before but it just seems hard to relax. Especially when palm muting. I wonder if my technique is wrong.
^ +1

At least rest for a few days, then when you're ready to get back to practicing, you just have to go slow, pay attention to what's happening with your shoulder and the rest of your body. If you can identify tension you can work on relaxing it. And if it hurts, stop. Remember this takes time.

I've had a similar issue in my playing as well, though maybe not as severe. If I don't warm up and just jump into playing my picking hand shoulder tenses up and starts hurting after a while. It's called sympathetic tension.
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Yes, this is a technique issue. Beyond visiting your doctor (especially if there's pain outside of when you play), you should take a close look at what you're doing with your right arm.

Ideal picking technique is pretty much entirely in the wrist. You can play really fast without even engaging the forearm or elbow.

Try this: sit down with your guitar in classical position, meaning the guitar is on your left leg and elevated, and get your right hand in picking position. Put your left hand on your shoulder so you can feel when it tenses up. Start picking really slowly with a metronome, like 60bpm or less, and take note of when you feel your shoulder muscles flex. If you can't get it to relax with a steady tempo, stop the metronome and try to do the picking motion extremely slow without your shoulder flexing at all. Controlling your muscles in this way may take some serious concentration if you're not used to it.

When you can maintain a steady tempo with relaxation everywhere above the wrist, you can increase the tempo and start incorporation the technique into music you know. Just keep a little bit of your attention focused on relaxation.