Because CTS is in the wrist, it is indeed the hand position that is responsible for it. In pianists, the most common cause is playing with thumb orientation, or generally, playing with the wrist at unnatural angles. CTS is one of the major concerns of pianists. Go through the threads on this forum, and also google for it. There are tons of web sites that offer a lot of info.
Here is a short list of what to do once you have symptoms of CTS:
1. Stop doing the activity that causes CTS
2. Make sure you actually know what activity causes CTS
3. Once the symptoms are gone, start to retrain, so that the symptoms won't show up again.
4. If you are having serious issues that don't go away after a couple of days of inactivity or are recurring, get medical attention.
Keep in mind that playing the piano might not be the cause for your symptoms, but it will aggravate it. Using computer mice, typing, etc. could be the main culprit. Whatever it is, it is always a consequence of misusing the body.