It is not clear why you have been given "THREE" EKG tests ( a stress includes an EKG reading at rest and exertion. Are referring to an echocardiogram? Your symptoms indicate a possiblity of aschemia (lack of blood flow to the heart) usually due to some occlusion of the heart vessels. A stress test should have been positive if that condition exists.
Checking with blood tests for electrolytes may be reasonable. Potassium defiency can be a cause of inverted T waves.
A T wave occurs when the heart is getting ready to pump again. The resting electrocardiogram (ECG) is an "inexact science", a two dimensional low-sensitivity recording of a dynamic three dimensional process, which offers limited diagnostics, and sometimes yields inconclusive or erroneous results/findings.
T wave abnormalities can provide added evidence to support clinical diagnosis. Except for hyperkalemia (low potassium), T wave abnormality alone is not diagnostic of any particular condition. The T wave MUST BE considered along with QRS and ST segment abnormalities also displayed with an EKG. Then an inverted T can be associated involving infarction (heart attack), chronic pericarditis (inflammation of heart lining), conduction block (an EKG reading would/should disclose...interference with electrical imnpulse), ventricular hypertrophy (an echocardiogram would/should show an enlarged heart).
The symptoms you describe could be heart related. If it is heart related, nitro medication would/should provide relief.