The simplest and most cost effective route you can take to minimize wrist pain until the pain subsides is to switch which hand you are using the mouse with. Switching from one hand to the other may feel a bit awkward at first, but it will immediately take all the strain of using your other hand off and allow it time to recover.
I personally have been using my mice ambidextrously for a number of years, and while there is a large divide between my writing skill with either hand, I can use the mouse equally as well in my left and right hands.
As other posters have mentioned, you should be aware that you have likely developed RSI and should speak to a doctor about the issue as well as integrate some ergonomics into your workstation.
If you haven't set up your workstation as follows already, I would suggest keeping the mouse just above your lap--the keyboard tray is a great location for it if you desk has one. This will put your arms and wrists in the most natural position possible for using the mouse.
You may also be interested in looking into alternative mice, such as trackballs, which have less strain on the wrist because only the fingertips move during operation.