There is a condition called as orthostatic hypotension or postural hypotension. In this change of posture especially from sitting to standing causes fall of blood pressure and that is responsible for the dizziness. The causes of it are;
1) Diabetes; autonomic nervous system which regulates our blood pressure is involved in long term diabetes and this causes the fall in blood pressure in changing posture. This is called as autonomic neuropathy.
2) Medication; Your medication for blood pressure may start giving this side effect.
3) Some heart related conditions.
4) Deficiency of fluids in body.
Following would be advised;
1) Lifestyle modifications, such as avoiding sudden postural changes
2) Sitting on the edge of the bed for 5 minutes before standing,
3) Calf muscle exercises,
4) Drinking 2 liters of fluids per day,
5) Avoiding hot environments,
6) Raising the head of the bed to 15-20 degrees,
7) Elastic compression stockings
8) Medication can be the culprit and should be considered either reducing the dose or stopping it.
9) Medical treatment; fludrocortisone, midodrine, erythropoietin, and pyridostigmine.
You would also be advised following investigations;
a) blood urea
b) serum creatinine
c) glucose tolerance test
e) complete blood count (CBC),
h) magnetic resonance imaging of the head to rule out Multiple System Atrophy.
Please feel free for your follow up questions.