I've had them at various times in my life from stress or trauma. Huge adrenalin surge. Heart races. Sweating. Shaking. Can't get a deep breath. Impending sense of doom and/or certainty that I'm going to die or something awful will happen to me or someone else. Nausea. Some people hyperventilate or faint.

Take heavy-duty fear, then square it three times or more. That's a panic attack. It's a very unpleasant event.