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You could be having problem with your blood pressure. As soon as you stand if you notice that you feel dizzy it could be Orthostatic hypotension. Get your blood sugar tested as FBS and PPBS. Consult a Physician to rule out this problem by measuring your blood pressure before standing and then after standing. Also more testes may be required to confirm. Drink plenty of water. Have lemon juice everyday. Avoid strenuous exercise. Get a checkup to rule out any infections which can also cause this. Do not delay.

Hello, welcome to HCM. You seemed to have an episode of syncope. Syncope or passing out can be due to various reasons including fever, fasting long hours, taking in too little food and fluids, low blood pressure, hypoglycemia, growth spurts physical exercise in excess of the energy reserve of the body, emotional distress, and lack of sleep. More serious causes of fainting include cardiac (heart-related) conditions such as an abnormal heart rhythm (an arrhythmia), wherein the heart beats too slowly, too rapidly, or too irregularly to pump enough blood to the brain. You must visit your doctor for complete evaluation. Wish you good health.