Persistent chest discomfort (6 months) and heavy left arm. 2 ECG and 1 Echocardiogram all returned normal. Active. Heartburn relief no difference?

Keep investigating. At your age, if you don't have a strong family history of early-onset coronary artery disease, & you don't smoke, have high cholesterol, hypertension or diabetes, your risk of heart attack in 10 years is less than 1%. You might consider pulmonary embolism, but your risk there seems low too. Keep your physician appointments & report any changes. Can help sort this out over time. Hope that helps.
Probably not ur hart. Given your age, unless there is a family history very premature heart disease it is likely not caused by ur heart. Just to be sure a maximal stress exercise test should be done, and if negative other non cardiac causes should be investigated. Irrespective of the findings any cardiac risk factors you may have should be treated and/or modified. Good luck.
??Blood testing-ER. Please go directly BACK TO THE ER and explain this situation. You will need a 24 hr. observation and serial cardiac enzyme blood tests. I suggest that you demand these blood tests and enter the hospital of a 24 hr observation. Please call 911 if your symptoms have been worsening. Call NOW!