Thanks for writing your query.
It will be helpful if you please mention whether you are on any medicines and any history of any illness.
It seems that you are having either postuarl hypotension
or vasovagal attack resulting in decreased blood flow to the brain.
You should get your blood pressure recorded in lying, sitting and standing position.
Also get yourself investigated for :-
2. Blood sugar
Also get an ENT checkup to rule out any internal ear dysfunction and a head scan to know the brain status.
Avoid immediate change in posture. Whenever you change in your posture, it should be slowly. This will not result in sudden fall in blood pressure.
Lying down with head at lower height than feet will help you in recovering soon from light headedness.
If the problem persists, get yourself examined by a Neurophysician.
I hope this is helpful to you.