Forearm pain is more likely to be caused by too much tension in the wrist and hand. (Of course, you should do what you can to reduce shoulder tension as well)
Can be treated by trigger point massage with a hard rubber ball.
My experience with trigger point treatment:
Interesting topic. I find I have severe wrist pain if I spend too long playing power chords in general. I think it must be related to the angle of my wrist. It got so bad I went back to open chords for a while to take a break. I am interested to hear how the discussion continues.
Jeff
I had some fretting hand wrist pain as wellā¦a few days ago. Resolved for the moment by massaging the muscles in the forearm that move the wrist.
I believe that it was caused by my switch to thumb muting. Either too much pressure, or too much wrist angle probably, but Iām still figuring it out.
Iām glad you backed off on your practice - repetitive stress injuries are no joke.
Iāve started to work on playing standing, mainly because my seated posture is really bad (as demonstrated on any of my AVOYPās 
). Strap adjustment will get it sitting at a good height for you although the way the guitar neck feels / moves is a little odd.
Having said that it really shows that your fingers really do know where they need to go regardless so would really recommend giving it a go. Plus it truly unleashes your inner rock god!! 
Skipping forward to Justinās lesson on playing standing up is probably the easiest way to get the details on how to do it effectively.