First of all, be a good consumer of healthcare, don't accept answers that tell you it's nothing. You have lived in your body longer than anyone else so don't let them bully you if they don't have answers.
A heart cath will tell you if you have blockages. If pulmonary hypertension
runs in your family, a heart cath or echo-cardiogram will give you some more clues.
is due to de-conditioning. How active are you? How many METs were you able to achieve on the treadmill tests. Have you worn a 24 hour event monitor? Do you have panic attacks? Sometimes an EP study will show abnormal heart rhythms. EP means electrophysiology--a test where they evaluate your electrical conduction system in your heart. Don't let them tell you that you are young and there's nothing wrong. Unfortunately, sometimes, modern medicine thinks that only old people get sick. Good Luck and feel free to email me if you have any more questions.