Pounding heart on standing and shortness of breath.sometimes chest tightness. i am 18 i have nrimal echo ekg and holter.whats causing it ?

Keep a Diary: You have done a good preliminary workup - if EKG / Echo and Holter are indeed normal best is to continue to monitor yourself - keep a diary / if you have a home BP monitor get some data at the time you are having episodes -It will hep to look for a pattern and specific triggers- decrease caffeine/alcohol intake. It may be reasonable to consider non cardiac causes - Anemia and Thyroid function etc