Actually, your story is told by many others who describe "sinking brain" feeling.
We believe this is a very special case of VERTIGO
that somehow is generated due to aberrant electrical signals possibly arising from the vestibular apparatus in the inner ear
which are then, processed by the part of the brain responsible to telling the person that they're in motion as well as the direction (up, down, side to side, etc.).
My suggestion is that you speak to a neurologist
(epileptologist most likely) and get tested with an MRI
of the brain (with gadolinium) and an EEG (at least 2-3 since only 1 may not be sensitive enough to pick up abnormalities).
There may be an underlying epileptic or epileptic-like syndrome that can be controlled with medication.
Hope I have answered your query. Let me know if I can assist you further.
Dr Dariush Saghafi, Neurologist