What would cause an odd stretching, vibrating sensation directly between breasts? lasts 5 seconds. no pain. fear cardiac issue.

Many things: There a multiple things that cause an odd sensation in your chest. Anxiety is a common cause and it seems you have had prior evaluations of your heart? Risk factors for a cardiac etiology of your pain are family history, smoking, diabetes, hypertension and high cholesterol. Follow up with your doctor regarding results of above tests.