It might be repetitive strain injury (RSI)
If your computer and/or mouse is positioned badly so your arm is twisted or stretching while using the mouse/computer that could cause the problem.
It may also be the way you are sitting (or the chair you are using), which can cause this type of problem.
I had a job a few years ago where I used a computer all day, and was getting backache. A lady from our medical team came to my desk and gave me all sorts of advice about where to place the PC and screen, how to sit, how to use the keyboard and mouse etc.
After following her suggestions all my pains went away.
Make sure you are sitting up in a good chair, and your arm is resting on a surface while using the mouse.
A search under "RSI computer" finds plenty of advice, Here is one
http://www.pcreview.c...titive_Strain_Injury/