Listen to the people talking about anxiety...

It sounds like you're having typical panic attacks(classic death feeling), the weed may have just been the catalyst to bringing them out of you... This happens to quite a few people and the fact that your mind works overtime thinking of worse and worse possible outcomes sounds about right. Smoking isnt for everyone and panic attacks are manageable, you should be okay in the long run. Do what you can to forget about the experience, harping on it will make it worse.

and never smoke again, it is not your solution.