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Breathlessness on exertion and relieved by rest is typical of heart failure
‘Something in the middle’ is most likely angina
At times, mild discomfort is misinterpreted as breathing difficulty.
This situation at this age is from heart – unless proved otherwise.
A single normal ECG does not exclude. Did they do blood tests? – Cardiac markers?
You have to see your doctor again. You need more investigations – like repeating ECG and blood tests / if normal, TMT (ECG during exercise) is a must / ECHOcardiogram
/ X-ray chest and so on are needed.
The best option will be to admitted for a day – for observation and investigation - and get things done
Wishing you speedy recovery