Trouble breathing, rapid heart rate, EKG revealed flattened t waves. Doc ordered echo consulted cardio who added nuclear stress test. Is this serious?

SOB . The description doesn't provide enough specific information to know the problem. There may be heart abnormalities suggested by symptoms and tests which require further investigation. You should ask the doctors with the information specific to you.
Probably not. Assuming you do not have risk factors, it sounds like anxiety, especially if you tolerate exertion well.