Your symptoms suggest that probably you are suffering from postural hypotension
..Postural hypotension is a condition in which subjects experience a systolic blood pressure drop of 20 mm Hg or more, or a diastolic blood pressure drop of 10 mm Hg or more, with or without an increase in heart or pulse rate, with or without symptoms, and within 3 minutes after standing.
There could be various reasons for your postural hypertension
and this includes...
....Ageing may result in vascular stiffness
causing reduction of baroreflex sensitivity of a person.
....due to use of anti hypertensives.
.. Use of diuretics may cause dehydration
...Cardiac disorder like cardiac failure
...diabetic auto neuropathy.
You must consult your doctor and get yourself examined properly to look for the cause and treatment.