So, you've had a cardiac work up, I assume, and nothing was found? Are they saying perhaps you are having panic attacks? They do very much feel like you describe and like you are dying. They can come on either with triggers or without. I would start journaling what is happening before the attacks to look for any pattern or triggers. And since you've had stress brought up, perhaps a good psychologist would be good to add onto your list of health care supporters to begin the process of looking into this. dietary issues such as low Vit D are easy to remedy with supplementation. So, do that. But if you have ruled out a heart issue, then I'd go the route of looking into mental health. good luck