I've had that startle-kind of response to objects- seeing negative or gruesome things out of what turn out to be not so- and it seems to be related to following negative incidents, negative happenings., more frequent or pervasive after them. Likely heightened anxiety, on edge, negative (sub conscious?) mindset.
I also know what you mean by the feeling of the bed shaking, which I've assumed must be my body shaking. It only occurs pretty infrequently, maybe once every couple of months for less than a minute. I've never considered it caused by anxiety, but that could certainly be so, and it's usually associated with exhaustion. Maybe low blood sugar or oxygen levels. I also like
@Rex 's possible explanation, as I get both migraines and severe sinus/ facial pain so I could see how it could affect the middle or inner ear. But it's a weird feeling, as it 'feels' like it's the bed, but it's not it's me. After a few times I didn't pay much attention, as whatever it is it's short lived. Like a big shiver/ mini-shaking, without thoughts I'm aware of causing it.
(I have been in minor earthquakes- 3.8 and such- but it causes objects around like ceiling lights etc to move (IMHE) , and they're not something common here).