I had a similar thing happen to me last summer. I would be sitting there doing nothing and boom, my chest got so heavy and i couldn't breath and i felt my heart racing out of my chest, but my pulse was normal. Went through the same battery of tests, the 24 hour monitor everything- they finally said that it was the muscles around my heart constricting due to panic or anxiety. They put me on Toporal XL. So that i wouldn't feel those affects anymore. I can tell you, first thing in the morning before i take my meds - i can still feel it- but i also struggle with severe anxiety. The days i forget to take the toporal all together are hard days. I can't do any physical activities without feeling like im going to pass out or have a heart attack. My family doctor is the one who figure it out. I guess just when you go back ask if it could be anxiety or stress related and what they think of trying the toporal xl.
MMahon is right, seeking counseling is really really good advice.
I've been through the same exact thing as yourself, I'm better on anti-anxiety meds and therapy but it turned out the chest and heart pains I was having were muscle cramps from my Fibromyalgia, I was absolutely positive I was having a heart attack, but now I know the difference and can recognize it without freaking out.
I'm not saying this is what is wrong, but am saying not to rule out that there are other reasons other than heart problems :) I hope this helps
If your doctors have checked you out and gave you an ok on your heart then trust that they are right and worrying yourself sick over it further isn't going to be helpfull.
You are young and the chances are very good that your heart truly is ok.
Has anything happened recently in your life, preceeding your anxiety attacks?
Consider seeking counseling or psychiatric help for the panic and anxiety if you haven't already. Depression and anxiety frequently cause the symptoms you experience and apparntly you realize that or you woulldn't be here.
Help is available so don't wait in doing what you can to get relief from your serious symptoms.
Good luck and let us know how you are doing.