Everybody knows that I'm a calm, controlled person; that I handle everything very well and am never anxious. Why in the world should I have back pain?  

Another reason. Back pain can happen for many reasons, most of them physical. Being a calm person does not preclude these etiologies of back pain.
No direct cause. There are many things that cause back pain; stress and anxiety are not one of them. It is said, however, that stress and anxiety can worsen conditions that one already has, but it does not cause that condition. Going to your doctor will help in identifying your cause of back pain and he or she can also suggest treatment options to help you alleviate the pain.
No connection. There isn't any connection between your personality and back pain. Most people experience back pain during their lives. It can be caused by a pulled muscle, arthritis of the spine, or an injury to the disc, which sits between the bones. It can also be caused by a compression fracture of the bones if you have weakened bone structure, called osteoporosis.