First and foremost, let's clarify a few things about a heart attack.
The chance of someone having a myocardial infarction at your age is essentially zero, unless you have had high cholesterol since you were an infant, and were or are excessively overweight and had untreated hypertension. A myocardial infarction is where your supply of oxygen-rich blood is cut off to the myocardium (the heart muscle) and because of this, if not treated promptly, can cause part of the heart muscle cells to die off. A heart attack in a way I guess, is basically the extreme of cardiac ischemia, which is lack of oxygen getting to your heart cells.
This is caused from a long time of chronically high cholesterol levels and other blood lipids (dyslipidemia) that get too high which end up depositing more and more plaque, very, very slowly over many, many years. If you were sedentarty, overweight, smoked daily and drank for thirty years, then yes you would probably have a valid concern about a heart attack. However, you are not. You're only twenty, you are active or were, but since you ran so much its pretty much a certainty you are not by any means overweight. And if you don't smoke and eat healthy, you have absolutely no reason to worry and probably never will if you take care of yourself.
The symptoms you are feeling in your chest, I can pretty much guarantee is from anxiety, its somatic anxiety - the feelings you are having are causing you the anxiety, and in turn you are now anxious about the feelings possibly thinking they are dangerous or an indicator of something organic. Also, the anxiety causes uncomfortable feelings that make you worry, and naturally, you worry about them and the symptoms get worse and it runs in a cycle. If you want to put your mind at ease, I recommend you do two things: The first is to read up on anxiety, you will be doing yourself a favor. Read up about the heart, learn some basic cardiology so you know about some of it. The second thing to do if you can, is to go see a general physician and talk to him or her about your concerns and if anything, they can give you a typical auscultation and maybe an EKG. That ought to put your worries to ease once and for all.
Now what you could be feeling, is a palpitation. A palpitation is nothing to worry over, they are completely normal and can happen to everyone. Sometimes people notice theirs, and some do not. A palpitation can be felt as a flutter, a thump, thud, double beat, a skip, a throb, or sometimes just an awareness of your hearbeat.
To deal with your anxiety, you need to just chill - just chill out and relax, make relaxation a sort of practice, keep yourself calm and you will train yourself out of being anxious. It takes time and persistence to do so, but it works. It would be advisable though to talk with a counselour or psychiatrist as well and see what they can do to help you.
If you have other concerns or just need to talk and get advice, this place is great for that and many are here to talk about it and offer help.
Best of luck, and remember to take it easy. ;-)
i believe that you may need to see a doctor that can prescribe something for your anxiety attacks ok :)