Just so you are aware, a lot of people go through something like this, usually in their mid to late 20's, usually heart, but often other organs are involved or some possible disease. Basically, it seems like it's some kind of coming to terms with the fact that you can die of anything at any moment, we live constantly on the edge, and pretend death isn't right in our face. It's not easy, human's have a strange detachment from that reality, but at some point in our life we usually get a little too close to it. This is probably where most of the anxiety from panic attacks come from. If you can convince yourself of the afterlife it instantly fixes it (or makes it worse, depending).