How do i know if i am stressed??????

I have been experiencing chest pain/tightness, i had an anxiety attack that made me feel like i was having a heart attack, i attended casualty had an ECG and the DR said that everything was fine. The symptoms continued and i attended my GP who told me this was down to stress. The thing is that i dont consciously feel stressed out, there is no particular event that stresses me out so i dont understand how the DR can say this. But when it happens i show the clic signs, distrubed breathing/fear/anxiety i become withdrawn etc. It just seems to come from nowhere, i get it at work sometimes however i also get it at night sometimes, totally random. Does anyone else experience this? is it stress? what is it????