What you could be experiencing could be any one of a number of paroxysmal cardiac events; multiple PVC events, VT, SVT. Once these pass,
your heart settles down, and appears normal.
I've gone thru the same scenario, even being questioned by a cardiologist as to whether I dabbled in drugs at all. After all, how could a 35 year old man with a background of competitive athletics have a faulty heart? I had nearly a lifetime of these events, and just persevered.