Thanks for writing your query.
After reading your post, it seems that your mother might be having orthostatic hypotension
also known as postural hypotension
In this blood pressure falls due to change in posture leading to decrease in blood flow to the brain resulting in dizziness
She should maintain her blood pressure. She should have a healthy diet.
Following investigations can be helpful :-
2. Serum electrolytes.
4. CT/MRI of head and neck.
She should not change her position in a fast manner.
She should do postural changes slowly.
If the problem persists, get yourself clinically examined.
I hope this is helpful to you.