You are having panic/anxiety attacks. I started getting anxiety attacks when I was 17 and for the first year it started as an extreme tightness around the circumference of my chest (like there was a big elastic band tied around all of my chest). you know what the funny thing is? I had no idea why I was getting them, my life was relatively happy and care free at that point. after a year, I went to a doctor who dismissed it as improper form while working out. She happened to be terribly wrong. as I still have attacks.
You need to see a specialist. Going through years with this disorder, I can tell that you actually don't have anything physically wrong with you. Your body is fine. Take a deep breath of relief. Go see a doctor however, because your racing thoughts and fear are trying to tell you something. Don't be scared, you'll be absolutely fine.
these forums have been helping me.
that happens to me all the time!! if i get a headache or feel dizzy, i think i have a tumor, and then i work myself up to a ful blown panic attack