Pain that you describe, occurring at rest and when you lie down, merits consideration of an esophageal reflux syndrome. It does not suggest a cardiac origin. You don't mention your age or eating habits, alcohol and coffee intake, or past history that might be pertinent. Nevertheless, it is noteworthy that such a problem might not be revealed by upper and lower gastrointestinal x-rays.
I would suggest that you contact your doctor about the persistence of your symptoms. There are diets and good medications to treat esophageal reflux and he may consider trying such an approach.
Please note that I am unable to diagnose medical conditions online.