If you turn your palms down and rotate your forearms outward until they are pointing straight forward (parallel with each other), you are probably at the outer limit of where you should routinely work. Keep most of your work inside that limit, in front of you.
If you are mousing right handed, for example, and your keyboard is so wide that the normal position of the mouse requires you to rotate your forearm so your arm isn't pointing inward or straight ahead but is now pointing outward to the right, you may start feeling pain under your right shoulder blade, which may become quite severe.
The problem is that people with this pain don't usually feel it when they are mousing. Perversely, they tend to feel it when they are away from the keyboard, so they don't make the connection. If you know of anyone with sharp pain under the shoulder blade, see if they do a lot of work with that arm rotated outward.