Not worry about a test, nervousness regarding social situations, tension related to a busy task list, or anything else related to actual real-world events.
It is untethered. There is no referent.
Suddenly, tension washes over of my whole body like rolling heat lightning so that I want to jump out of my skin, heart first. I thrash my limbs, I scream quietly. I know that it will pass.
I also know that it will return, sometimes preceded by the thought that it hasn't happened in a while, or that this would be a bad time for it. There is no problem to trace back to and solve. It is a fact of my life.