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It is very likely postural hypotension
- momentary delay in reflex mechanisms is the usual cause.
It could be POTS (Postural Orthostatic Tachycardia Syndrome
There may be several other reasons - like electrolyte disturbances, anaemia, arrhythmia
and so on. Without idea of your age, personal habits, co-morbidity and family history and so on, it is hazardous to go on guessing more! Only examination and investigation can clarify.
See a doctor
BP needs to be checked standing and lying
Hormones and tilt tests are usual in these circumstances...
Wishing speedy recovery