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symptoms can arise when you have over-exerted yourself, when your blood sugar level is low, when you are too anxious, when your Hb% ( r.b.c ) in blood is low, inadequate oxygen
supply to your brain, low blood pressure or a combination of some of the factors illustrated.
To avoid such a feeling eat nutritious diet, include green veggies in your diet, keep yourself well hydrated,do not over-exert yourself, don't stand up too quickly,inhale plenty of fresh air, get blood tests done to detect you blood sugar level, haemoglobin
, etc, get clinically examined to check your fitness and get treated for low blood pressure.
See a doctor, if symptoms repeat. Don't worry you will get fine soon. Take are.