Could be a few reasons:
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Poor posture. I recommend watching this video for proper posture (Ignore the clickbait title). https://youtu.be/EqeDWH4kcME
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This one is most likely: you likely are gripping your mouse too hard and/or tensing your arm. You want your grip to be relaxed.
In this video, I'm going to go in depth on the physical mechanics of aiming, so that you can learn how to sit more properly reduce pains from gaming in your back, neck shoulder, elbow and wrist pain from gaming. This will help you reduce gaming pain, and become a better player. This will help you sit in a way that increases your skill ceiling. Y...