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you have can be due to many reasons right from tiredness
and fatigue;anaemia;low electrolytes;postural hypotension;anxiety;error of refraction
(vision problem) etc.Please consult a doctor and get blood pressure checked and drink enough of water and better add electrolyte powder to the water and you should feel better.Also get your haemoglobin
checked and if that is low get medicines prescribed by a doctor for increasing the haemogobin.