Sudden cardiac arrest is very rare.
Considering that you are young and that your heart has been thoroughly checked out, your risk is essentially zilch.
The 'thuds' you feel are very likely premature ventricular contractions, small, early beats that everyone gets--but which only hypersensitive people really feel.
You sound as though you flirting with a pretty serious case of 'cardiac neurosis,' which you can google.
At this point, you have two choices:
1. Let your enjoyment of life be paralyzed by a fear with virtually no basis in reality.
2. See a therapist or shrink to learn how to minimize your fear--and get back to a normal life.
Its rare, happens to people with heart abronmaloties. In my case its weird, Ive had every possible heart test come back negative. MRI, echocardiogram, stress echocardiogram, stress tests, EP study, and countless blood work and xrays. Still a holter monitor caught an episode of ventricular tachycardia. With seemingly no cause. Ive been terrified and paranoid all of my life about having heart problems. Since im a hypochondriac, it sucked really bad. Now that they actually caught something, im terrified and dont even leave my bed. Im only 23, no family history of cardiac problems or sudden death or anything. Have done sports all of my life, except this last year cause i stopped from the fear of dropping dead. I dont understand how i got the ventricular tachycardia. Instead of one skipped beat, or pvc, it was 22 in a row. Which is crazy. Im terrified.
If I were you I would read ana re read the post above from
All thats in it is exactly correct.
Its time to live life and chill!
Do as Achillea says, its the right advice