If it gets constantly sore.... STOP PLAYING, until the pain is better.

Another tip is that you should watch how you are holding your wrist when you play. Some bending is normal, however the less you can bend your wrist, the better. Don't think of it as stiff, but don't play your guitar down around your knees either. As you play just ask yourself if your wrist is in alingment with your fore arm. Remember... NOT A STIFF WRIST, BUT RATHER, AN AWARE AND SUPPLE ONE!