As someone who has lived through 3 incredibly painful bouts of tendonitis, both in my left arm and right arm, I thought it important to pass on some information that has helped me deal with this in the past.
It is important to understand that playing the piano can cause serious injuries such as tendonitis and carpel tunnel syndrome. Both these ailments are known as repetitive strain injuries. The constant up and down, side to side movement of the arms and fingers when playing piano for long periods of time is what causes these injuries.
This is common among piano players. In fact, it is so common that there is a special clinic in Hamilton, Ontario called the Musician