I've had "gamer's arm/shoulder/elbow" ever since... er... (looks it up) since Minecraft first showed up a million years ago.
I went too far using the mouse, you see. I've got free time, all the time, and Minecraft was something special, and it uses the mouse a lot, and.... yeah. Once it hit, it REALLY hit.
I actually had a period of about... 2 years, I think it was, where I just outright could not use a mouse for gaming. I rigged up my Playstation controller to emulate the mouse & keyboard for games that had no controller support. I tell ya, my control scheme for that game specifically was like the worst one you could ever find. But I got so stuck to it that I still use it to this day even though that game now supports controllers properly.
These days, I wear an arm brace at all times and I use a vertical mouse (and in my opinion, vertical mice should be the standard... seriously, try one, they make normal computer mice seem like a design error) and I go to physical therapy, which keeps the pain at bay. I've improved a lot... like, A LOT... but the condition isnt going away. I still have to be careful and there are certain types of games I wont touch (anything that requires I click a million times). I often still use the controller to emulate mouse/keyboard depending on the game... fortunately Steam comes with like 5 billionty accessibility functions these days, so it's very easy.
Controller shape also matters too. I've got an Xbox Series X, and the controller that came with that is the best one I've ever seen, which is saying something. In terms of like, comfort and ergonomics and all that.
I always cringe a bit when I see something like pro gamers playing freaking Starcraft in national tournaments or something and clicking 10 billion times per second, knowing how much practice goes into something like that. I think quite a few of them will be regretting that later on, but they wont have any clue what's happening until it hits that snapping point. And if it does, that is pretty much guaranteed to end their tournament career.
And I tell ya, tendon/nerve/joint pain can be overwhelming.
Granted, different people are more/less susceptible to it... I overall have a tendency towards chronic pain. My first incident was actually in my lower back, I still remember I was just walking through the house one day, and then very abruptly I was on the floor. With gaming, I was never the sort of player to do like, marathon sessions... I dont sit still well enough for that... but even like, 3 hour sessions can add up, particularly when you're also doing other computer/whatever tasks in the same day.
Too much gaming over the last two weeks and now my right wrist and thumb are killing me. Naproxen is not touching this sore pain.
Two things to try: Advil, and ice.
These arent for getting rid of the pain though. They are for controlling inflammation. It's the #1 thing that's been drilled into my head over years of physical therapy and doctors and such.
Unfortunately, this type of pain is very hard to cancel out via meds. When I was at my worst, stuff like Valium or Vicoden would do nothing, and I was taking like 4 Advil every 6 hours. Though that was for the lower back pain, not for my arm. But tendon/nerve/joint pain is just like that.
Maybe consider seeing a doctor.
And take it seriously. You dont want to end up where I'm at. It's worse than how I describe it.