For the past year or so I’ve struggled with identifying this issue.  Sometimes it feels like a stomach spasm and sometimes it feels like a THUMP following immediately with a surge of adrenaline.  Either way, there’s always the adrenaline rush.  I’ve had anxiety for a looong time, had 3+ EKGs (nothing unusual) and exercise stress tests (nothing unusual again).  I’m certainly not new to panic attacks, etc. but this is unique in that it’s there and gone in a flash.  It mostly happens lying down going to sleep or sitting idle at my desk.  Almost NEVER when I’m active.