Scarring can be from a previous lung infection. The last chest x-ray you had was on Apirl 12, 2007? They can look at that and see if the scar was there then, too. If it wasn't, they probably want to find out what caused it.
Do you have a current lung infection that is causing the chest wall pain? Do you have pneumonia now? Did they give you medication for it?
Do you have a regular doctor you can see to follow-up on this issue? If so, make sure the chest x-ray is sent to their office. If the scar cannot be explained, you should have further testing done to make sure it is really a scar and not something else and that something isn't causing it that you still have.