In my opinion there is no need to panic.
In view of normal X ray
& blood work & nothing specific on EKG the probabilty of finding any significant heart condition is remote.
Earlier also cardiologist did not find anything wrong with hear.
More likely is possible anxiety
or emotional upset, any break-up ?
Also worth exploring is issue of any substance/drug abuse.
If cardiologist gives ok (I am pretty sure of) , consider seeing a child psychologist