Hi... can understand your concern...As per your query about low blood pressure can be due to less water intake and condition called as hypovolemia in which BP becomes low or may be due to fasting ,dieting or it can be hormonal imbalance..Untill you visit the physician you need to take salty diet and tea coffee will help with diet containing more carbohydrates..Drink plenty of water and juices to maintain electrolytes in body and avoid prolong standing.prevention to be taken to avoid such conditions and any underlying cause should be treated first like hypothyroidism or any cardiac anomaly etc .I hope the information helps you.Take care.