Thumping heartbeat and weird tired feeling like i might fall to the floor. a few seconds of breathlessness at end of episode. what could this be?

Visit your Doctor: It is difficult to say exactly what this could be without evaluating you in person, however if you are experiencing these symptoms persistently, i recommend that you see a cardiologist even in spite of your normal ekg. Perhaps a holter monitor or stress test would give your physicians more information. If you experience chest pain or pressure, shortness of breath & dizziness visit your local er.