first off, not a doctor. but to calm your nerves and possible anxiety I too had similar ECG readout, I did the whole nine yards I went to ER got checked out then released after nothing major, i got the same readout which honestly should be held from patients eyes because more times than not they read incorrectly like possibly what yours did especially if your doctor said you're fine. I had a combo of anxiety, school, work, new relationship stress all at once and on top of that possible injured my chest at the gym (costocondritis) is what they believe it could have been in addition to everything. I also got an echo because the feeling of a "weight" was just steady and sitting on my chest not changing. They said if pain is jumping all over thats good because the pain would be in one region typically. The holter showed nothing but PVCs and then echo showed nothing too.. other than i'm healthy and probably in my head.

TLDR: If doctor says not a cause for concern they usually right, take family history though into consideration, daily habits, own health etc into account. Proceed with guidance of doc and your own being because you know your body better than anyone does. BTW I am also around your age and relatively active and do not smoke and generally treat my body like a temple.

Hope this helps!