number one - go see your GP, if there's any problem with your heart itself, best get to it ASAP.
I just did a google on "Flat T wave" and came up with a few things, namely thiroid abnormalities, as well as the more alarming stuff. But as you're young, fairly fit etc (smoker?, drinker?, how's your diet?), it's probably entirely treatable.
Not that I'm qualified in any way of course, but when I was 16, and in the Scouts, I took part in a raft race, part of which involved carrying the raft for about a mile. I wasn't that fit, pushed myself too far, and ended up damaging my heart, or rather its firing sequence mechanism. It was rather scary at the time, not least for my parents, and I ended up learning quite a lot about cardiology. All perfectly healthy and normal now.
Don't panic.
SHK