Thanks for writng your query.
After reading your post, it seems that you are having low blood pressure and it falls more on sudden change in posture like when sudden standing. This is called postural hypotension
You should get your blood pressure monitored.
Following investigations will be helpful :-
3. Serum electrolytes.
Also take care of your diet. Fresh fruits and vegetables along with rehydrating fluids will be helpful.
If the problem persists, get yourself examined clinically.
I hope this is helpful to you.