If your heart, Kidneys and Liver are fine...
This might be something to do with your adrenal glands, if you diet is healthy and you are taking in plenty of nutrients and especially minerals/Celtic/Himalayas sea salt. Also if you're liver is fine and you are not due a detox, i honestly think this is down to having weak adrenal's maybe due to chronic stress....
When your adrenal glands are weak, you can have low blood pressure! and serious fatigue.
You could just play it safe with a supplement like this, and also be sure to eat plenty of foods which contain magnesium. Your adrenal glands use allot of magnesium to function and Vitamin C. Eat cashew nuts, and green leafy vegetables....
http://www.adrenalfatigue.org/produc...enal-c-formula
: Also avoid drinking, caffeinated drinks even tea's. Caffeine stimulates your adrenal glands to start pumping out chemicals and hormones, people think its caffeine that gives you energy. Caffeine is just a stimulant, which means its just a co-factor to something metabolically that your body still have to do itself!! Caffeine will over-stress your adrenal glands if they are weak.........