When a joint is used a lot such as your wrist while playing the guitar, this can cause pain from a few different possible causes. Tendonitis is certainly one of them. There are a group of tendons that run through the wrist and attach to muscles that control the hand and fingers. In some cases when these tendons are used a lot, they can become inflamed which can be painful. Thus, I agree with your doctor
that this could be a tendonitis. However, you are also correct in that it could be something else. Carpal tunnel syndrome
is a condition where a nerve (called the median nerve) gets trapped and compressed within the wrist itself from inflammation in the surrounding tissues. It can cause wrist pain, but most people complain of pain and numbness in the thumb, and first two fingers. Finally, it is possible that you are experiencing pain from arthritis in your wrist joint. This is inflammation that occurs between the bones of your forearm and hand. All of these possible causes can be ruled in or ruled out by a simple physical exam by an experienced physician. This should be a problem that can be handled by your primary care physician
. I suggest that you schedule an appointment. Good luck.