[occoder]

Yep, it's almost always the mouse.

Many people, developers included, use the mouse to point and click when they need to change the caret position while editing text. If you do enough of that, the amount of precisely controlled movement involved quickly add up to repetitive strain injury on your hand/wrist. I've seen colleagues in their 20s/30s suffering from this problem, to a point that they needed to use the mouse with their other hand.

I'd recommend using the keyboard to navigate within a document. It's going to take some getting used to, but it'll be best for your hand health in the long run. Meanwhile, switching to a trackball (remember those?), a trackpad, or a trackpoint mouse, will definitely alleviate the symptoms.