Welcome to healthcare Magic!
Cardiac cause if important and you have done the right thing in getting it rule out as a cause for your problem.
Now another two most important possibilities still remain.
One is as you have already mentioned the acidity problem and the other is a pleurisy
that is a fluid and inflammation in the lung.
To rule out pleurisy you need a Chest Xray.
If nothing important on CXR
then you have acid peptic disease
and would get great relief if you regularise your meals and take tab pantoprazole
40mg once a day empty stomach in the morning for 45 days.
Wish you a great health!