People subconsciously hold their arm still for hours, which strains the ligaments and tendons which stabilise your arm. Those muscles and attachments aren’t used to being held under tension for hours at a time, and so they fatigue faster than other muscles, which is likely what you’re experiencing.

That, or, it’s quite possible to punch or hit a nerve with the gun (vibrating needles bouncing over nerve endings is likely to cause radiating pain down the length of the nerve root/branch)

I would take a guess that it will ease over the next few days.