I understand completely, I've got carpal tunnel and although I don't get the pain anymore, my wrists do get tired and/or stiff.

I had tendinitis in my shoulder for 10 years. It's been taken care of but because it's been so long I'm probably gonna feel the effects for a loooooooong time.

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

It sucks, doesn't it?