You know I really feel for you, because I get this fear a lot myself. I have high blood pressure and worry about that, and know about the pressing sensation on the chest like the heart had stopped you talked about.
Here is the good news. If your under the age of 25, a heart attack is virtually unheard of in the medical community. Also, if your blood pressure is good, (diastolic consistantly under 90,) then thats another point in your favor.
To me if your that worried about it, call your doctor and request an ECG be done. They just put stickers on your chest for a bit and look at the rhythm of your heart. It takes less than a minute, and will tell your doctor everything he/she needs to know about your heart pretty definitively.
Now as far as immediate concern, you're not in any danger unless your face becomes covered in sweat, you feel the classic strong pain going from your chest into your arm, at that point i'd seek medical attention. But at this point, you've given me no reason to think you're in any immediate danger. :)