i experienced it once when i heard a very bad news that struck me like crazy. like Luthienne, after i experienced it, it just comes out of the blue whenever the thought comes up. mine is of very mild nature that i don't consider it as a diagnosis; i pretty much ignore it for the most part. i experience the most common signs like feeling sweaty, increased heart rate/palpitations, chills...etc.. i guess one factor that triggered it is the fact that i was a non-stop worrier before.. i worry about stuffs even the slightest things. every little thing i encounter that needs to be worried about, i worry everyday until it gets solved.