Age 52 , Good Health. Had Chest X-ray In May.

Age 52 , good health. Had chest x-ray in may. Had another in july thought I had puenomia. The one from may is fine. Second one showed up different. Had a CT scan show it showed a 1centimeter mass on left lung. Going to lung doctor next week. What are some of the causes. X-ray changed from may to July.