There are many causes of clubbing of the fingers. Clubbing can be due to lung diseases, heart problems, liver diseases, thyroid dieases and malignancy. In some people, the cause is idiopathic ie. there is no undelying cause.
Clubbing is often caused by chronic hypoxia or chronic low oxygen levels. Smoking may cause lung diseases like bronchiectasis which can cause clubbing and it is advisable to quit smoking if possible. While lung cancer can cause clubbing, this would be an uncommon cause in your age.
The diagnosia of clubbing requires a good history and examination to determine the underlying cause. Some investigations may be needed to rule out an underlying cause. You should let your doctor know of your concerns so that a proper assessment can be carried out for you.
