I'm not a doctor, but i do have a BA in psychology, and it certainly does not sound like panic disorder to me. Seafrog, i guess it would be best if you talked to a professional. It COULD be OCD or a form of it... I can relate totally to the dying-in-my-sleep thing, I went through a similar thing during one of my anxiety phases. I have had different worries/amxieties/obsessions, but I have been diagnosed with OCD. It is controversial if hypochondriasis should be seen as a form of OCD or not, it is certainly related. Sometimes you might hear the term "OCD-spectrum-disorder".
it does sound like an obsessive thought, the "I might die" thought. I cna just kind of feel the anxiety just thinking about it. Do you have certain rituals you perform sometimes, certain thoughts you think you should or should not think? fears that you might die BECAUSE of something you or somebody else did (it sounds like that from your post).
Tell us more, ok?
And remember, there are good ways to treat these things!