I have chronic anxiety. 
Sometimes when I’m sitting and relaxing after my day, I’ll be completely into what I’m doing and not stressed but I’ll suddenly get this feeling that overwhelms me. It lasts for only a split second, but it’s intense, like I’m about to die. It sends a literal shock through my body and then I get really scared. Only for a split second, though, and then it’s like my body catches itself and everything is back to normal. DAE feel this from time to time? Happens to me once or twice a day now.