Here's the thing: my left wrist is hurting. I've read lot's of threads about this but, as far as I know, none of them applies to my case.
I noticed that the "natural" or "most relaxed" position of my left wrist is a little spinned unclockwise, while my right (which has never hurt) can adopt a "horizontal" position with no effort at all. I'm almost certain that the pain in my left wrist is due to the effort I make every time I try to get both hands straight.
So tell me, is this poor positioning of my wrist due to a bad position of my back, arm, shoulder or anything else? Or is it possible that I was born with a slightly "spinned" left arm? I tried not sitting so close to the piano, as some suggested, and it helped a lot, but not completely.
Have you told your teacher? No part should hurt when you play the piano. In fact, most of the time you should feel almost no tension at all in your hands.
There are two possible things, an injury not due to your playing style, in which case, seeing a doctor and resting will help; and the second is that you have develop a style that is not suitable. In the second case, you will have to start changing your playing style. My advice is to not ignore it but to address it. Learn some movement. The problem with this is that, the easiest way is to actually see someone with good movement doing it, and imitating.
Here are some tips,
1. sandor book has information in about extremities of hand position. there are basically three types hand movement -- left-right, up-down and rotation. To each of these there is a comfort zone (well within the movement). just check when you are playing that you do not stay near the extremities of each of these motions for extended periods. your hand position should be in the comfort zone most of the time.
2. you should play with every joint of your fingers. The wrists should be supple. to a certain extent, you can think of every joint of your hand as loose springs and the action should comprise of the extension/compression of each of these springs. In general, no one part should be ridgid.
3. Concetrate on playing a single note at a time, looking at your movement. The moment should comprise of a conscious effort to play the note and after, a natural recoil action. Again, all parts of your digit should work simultaneously.