You didn't mention what posture you are using whilst playing, but my guess would be that your left shoulder is supporting your arm too much. The neck of the guitar needs to be such that, when you are seated squarely with back straight, raising your left hand, palm up, by bending only at the elbow (not at the shoulder joint), your hand will engage the guitar neck. Some shoulder joint movement will be required, but if you find your elbow moving far away from your torso, you will be using the shoulder too much. Each person is different, so try using different posture aids - foot stool, guitar supports etc. You can also shift the guitar away from the idealized "classical" position if that really doesn't work for you, but don't go too radical. Avoid anything that takes your back out of the square over the hips position.
Last edited by hemiphractus on Mon Apr 25, 2011 12:56 am, edited 1 time in total.