When i stand up from bed i sometimes feel blackout and fainted which lasts for 5-20 seconds. what could be the reason and treatment for it?

Hypotension: Your description is consistent with postural drop in blood pressure which may be due to medication rxtns, anemia, or dehydration, or dysautonomias. Explore with your doctor the precise reactions and uncover the precise etiology. In meantime, going from supine to erect, do not rush, go slowly, and consider wearing ted stockings to protect against blood pressure drop.