I also suffer from general joint pain, and I find my wrists get really quite painful quite quickly when I type and particularly when I write.
You will know yourself whether or not you do either of these or any other exercises which are putting excessive strain on your wrist.
Carpal Tunnel sounds like a possibility, or it could be a build up of fluid. Does it make a clicking or crunching sound when you bend it?
Since it's been ongoing for quite a while now, seeing a GP may be the best thing for you, rather than putting strain on it and possibly making it worse.