For my, it was 100% my mouse. I had my mouse off to the side, and I realized after debilitating wrist, elbow and shoulder pain that it was because my wrist was at an awkward angle all the time because of my mouse.

The reason for it is obvious when you look at it. When you write in a notebook as a student, your arms are almost crossed and your hand is bent at an angle relative to the table. You can maintain this position for hours a day, years on end when you're writing notes. When you're using a mouse, your arm is bent outward and your hand is parallel to the table surface, which twists your wrist. That ends up pinching a nerve in your wrist, which causes all these other problems.

I switched to a pen mouse (back in 1997) which immediately fixed the problem. However, the pen mouse broke after a few months, so I went back to my regular mouse, but I first put it in front of my chest, in front of the keyboard, and I cup the mouse so that my hand is at an angle to the table. My arm from the shoulder to the elbow is resting against my chest, just like when I write on paper, and I don't let it get off my body. When I type I move my mouse out of the way, and when I use the mouse I move the keyboard out of the way.

It's been 22 years now without any repeat of the problems, so I know at least for me this fixed the problem.