love it how my right arm randomly gets tired
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That's actually what mine started out as (but in the wrist obviously) and progressed into carpal tunnel. If I had to pick a singular cause, it would be playing Elder Scrolls Online and trying to get high af DPS. Sadly, I don't play it anymore because the pain and discomfort just isn't worth it.
EDIT: probably reading manga/webtoons on my phone doesn't help either