No, it is not normal therefore, it could be a sign of an injury or a disease. The pain that you are experiencing can be caused by injury to the knee cap cartilage or by a type of arthritis. You should consult your doctor for a more professional diagnosis.
I have the same problem as you. I have gone to two different doctor about it, one said it was growing pains, and the other (after doing an MRI) could not find anything wrong. So I have just learned to live with the pain. You should still go have it checked out by a doctor. Hope he can help you! ;)