Too much fapping?

But in seriousness you might be strumming crazily or something, do you do anything wierd when you strum?
I sometimes get it if I pick fast without stopping for sometime. Do some shoulder exercises about every 5 mins.
Are you picking with your wrist? If you are, then I have no clue. If not, there's your problem (most likely).
Could be from tensing up. I pick from my wrist but I tend to tense up when I speed up and am downpicking. I'm trying to avoid this, but after doing my muscles hurt a bit.
yeah I pick with my wrist... I usually anchor my picking hand on the guitar, that usually hurts less or barely anything, but it does restrict my picking movement. When I do not then my shoulder hurts ;( NO i have not been fapping ;P If you guys want Ill post a video of my picking action
for a really short time that happened to me because i was tensing up practicing fast down picking. I got over it in no time though.
I'll tell you right now that its most likely because you are tensing up your shoulder while playing. Try to relax a bit more
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I'm pretty sure too! I'll try to find time to get a good video to see, but yeah... are you supposed to rest your arm on your guitar? (somewhere... on top of the bridge/pickup)
Well that depends. It's alright if you're not anchoring (read the stickies).

You probably have a lot of tension though. Try to really focus on where the tension is in your body, and relax the parts you don't need tension in and reduce the parts where you have too much.