Thanks for writing to us.
For all practical purposes this seems to be anginal pain. A retrosternal pain
(behind mid chest) which comes on exertion/ exercise and relives by rest is likely to be angina, unless proved otherwise. You must see your cardiologist at the earliest. He will do an examination and will let you know the future course, which depending upon your examination, will involve ECG/ECHO/ Stress
Till then do not exert much. Avoid strenuous exercise till then.
wishing you good health