I had my first panic / anxiety attack when I was 21 (a looong time ago now!).
My heart was racing and pounding, I felt I couldn't breathe and that I was certainly going to die.
At that time I would also have described myself as "not prone to panic attacks and don't have a stressful job" - in fact I was a student at the time, and it was the middle of the summer holidays!
You would be surprised what forms "stress" can take, where it can come from and what it can do to you!
I am not a doctor, so you must go and see your own, but (based on my own lengthy experience) it does sound possible that you are experiencing some form of anxiety / panic attacks.
If so, you have nothing to worry about - oh wait, you already are worrying about worrying about worrying about...
FBW