Seems that your blood pressure drops when you suddenly stand up. This is called as
postural hypotension. It is common during
pregnancy, particularly if associated with
anaemia. So get your blood checked and take measures to correct anaemia if found. Eat good amount of
proteins through eggs and iron by medicines.
I would advice you to get you blood pressure checked up in the following positions:
Lying down on the back
Lying down on left side
Sitting position
Standing position
All these readings should be taken without loosing much time. They will help the diagnosis.
Meanwhile some advice to avoid these symptoms:
Whenever you lye down, lye on left side.
While getting up, if you are in any other position, turn to left side and get up slowly.
After you sit from the lying position, wait for some time before you stand up. And do all these movements slowly.
This phenomenon is not threatening. Take care please.