Brief Answer:
possible causes explained below...

Detailed Answer:
Hello there,
Welcome to 'Ask A Doctor' service...
I've gone through your query in details...
I can understand your concern...

Well, there could be a few possibilities in this scenario. Firstly, it could be due to a sudden drop in the blood pressure due to the postures (postural hypotension).

I'd suggest you take the blood pressure reading at different times (morning, afternoon, evening) at different posture (after lying down, getting up from the bed, sitting) for a few days and keep a trend of readings. If the blood pressure reading noted fluctuating more than 15-20mmhgSystolic (upper level), and 10-15mmhgDiastolic (lower level) then the diagnosis is confirmed.

To prevent this, drink ample amount of electrolyte-rich fluids and water before going to bed and don't lie down or get up suddenly. First, try to sit down for a while, then lie down/get up slowly with some support.

Another possibility would be benign paroxysmal positional vertigo (BPPV). In order to rule this out, an ENT check-up would be required. Some other possible cause could be the electrolyte imbalance (Na, potassium notably), or decreased blood flow through the carotid arteries of the neck during those certain postures.

As of now, the blood pressure readings should be checked as explained, if it's ruled out, then other possibilities could be look out for.

Please discuss these with your treating Physician.
Let me know if I can assist you further.

Take Care
Kind Regards

---