It's very frightening isn't it Smowball? I used to get them quite a lot. You think you're quite relaxed then suddenly it hits you. My doctor said when stressed or anxious, even if you think you've buried it, these feelings of panic will eventually rise to the top if not dealt with properly. They will break through. Have a chat with your doctor, who will explain things to you and tips on how to deal with them. Like someone else mentioned, they will not kill you and will pass.