Shaking but I'm not

why does it feel like a earthquake only when i sit still or lay down can you explain this to me

I’ve felt this sensation multiple times. For me it’s as though there are roadworks nearby and a worker is using a drill of some sort. I honestly don’t know why this happens. Does it happen every time?

I get this when an event in my life triggers my ptsd. The shakes are part of anxiety. My attacks last for days.
Scary feeling. Does this occur when you are stressed or frightened ?