What are the chances a lung or heart condition can be missed by a stethoscope exam? dr says normal but i get episodes of chest tightness/short breath

History, not exam: 90% of diagnoses are made on the basis of historical information. The exam is very secondary. The stethoscope is an insensitive instrument but your doctor no doubt made his/her assessment based on your symptoms, risk factors, demographics, and the incidence of disease under consideration. You self-report a "generalized anxiety disorder" which is a common cause of your symptoms.