Thanks for your query...
Did you have such episodes before? May be you are having orthostatic hypotension
Orthostatic hypotension is defined as a fall in systolic blood pressure of at least 20 mmHg and/or in the diastolic blood pressure of at least 10 mmHg between the supine reading and the upright reading.
So get a clinical examination done by your doctor and get it diagnosed.
Update here about your condition.
Hope this helps