Copying from my Craigslist ad -- I know this has nothing to do with computers, but it's free!
"This is an almost new, full size Aerobed that was purchased quite a while ago from Costco. I've just unboxed and inflated it, and found that I'm having some sort of weird allergic reaction to the...
IMO, you should see a doctor about it if you can, rather than just getting a new mouse. There could be exercises you can do, better posture, etc. Sorry, that doesn't really answer your question.