well id say thats stress - if it was anything else youd suffer other symtoms - tight crushing pains, pains in jaw arms upper abdo maybe back, sweaty or clammy, shortness of breath
believe me i know exactly how you feel! i have chosotchondritis - which is basically inflammation of the cartilidge in ya rib cage - my chest feels tight and painful most of the time - and i was paranoid as hell when it all first started - i had a few panic attacks convincing myself it was a heart attack - but i realised that without any other symptoms, and the fact that i "felt" short of breath, but actually wasnt reassured me and since, ive been fine - obviously still get the tightness but im so used to it its normal now...
its differenet for everyone - but its all about your own coping stratergies and changing your own thought processes - you need to try and speak to someone, doesnt even have to be a doctor, close friends can help too :)