The sequence you describe is palpitations, shortness of breath, dizziness, headache and feeling shaky. Palpitations (which are subjective sensations) are due to various medical problems, but one has to make a diagnosis. A resting electrocardiogram may not show the problem.
I would suggest that you discuss with your doctor having a 24 or 48 hour electrocardiographic study (Holter monitor), preferably hooked up while you are having the palpitations.
I don't know how a tilt table study would help, nor am I sure why a CT scan was done and what it assessed. At your age coronary heart disease would be quite unlikely. Please be aware that I am unable to diagnose medical conditions online.