You're reaching too far for your mouse.
If your mouse is too far away, your arm may be constantly outstretched while reaching for it - placing stress on your shoulder, upper back, and elbows. Make sure that your mouse is positioned within easy reach. You may also want to consider a mouse tray or a keyboard tray that's large enough to hold your mouse, or a trackball to minimize the distance your shoulder has to travel. See Mouse Setup and Usage or Mouse Overview and Accessories.
You're leaning on your elbows.
Many people don't even realize that they're supporting themselves with their elbows until a friend or coworker points it out. Make sure you're using proper sitting posture, though if this is how you commonly work, you may also want to consider padded arm rests on your chair.
You're overusing your mouse, keyboard, or calculator.
Striking the keys too hard can lead to problems can lead to painful inflammation of the tendons (known as Tendonitis). Also, make sure to take frequent "mini-breaks" - these can be as short as 15-30 seconds, but will help give your muscles the needed chance of pace. See Keyboard Setup and Usage or Mouse Setup and Usage.