How can something you love so much lead to pain? Any motion that you do repeatedly and improperly can lead to RSI, or Repetitive Stress Injury. For actions, such as needlepoint, RSI is often in the wrist, otherwise known as Carpal Tunnel Syndrome.
In extreme cases surgery can be needed.
The Needlework site over at Craft Gossip has a great post on actions you can take to help prevent this.