Irregular beats

i am 23 years old, i was a smoker since i was sixteen not a chronic one though, i quit 2 months ago and im not planning to go back, now i feel this thing on the left side of my chest right above my heart a little pressure as if there were something there also when exert my self i get loud and fast heart beats i get a tension in my temples and my head and i feel as if im gonna have a heart attack this feeling i have does not go away it just gets mild and im able to do things but at certain times during the day i feel the above symptoms and it is very scary i have done ecgs, echo, holter they showed good the doc said i had a low heart rate because i play a lot of soccer and there was a mild pause in between but not to worry, i also feel a mild sore throat sumtimes, i really need some advice on what this can be.

[lazydog]

Get thyroid checked.  Can cause those symptoms and also the sore throat.  
Get copy of your results and check for yourself.  Often told "normal" when in fact labs use old ranges.
...Julia