There are a number of possible causes which include, bad circulation and or low blood pressure. Below average metabolic rate, the rate at which your body works to digest food and the like. Low iron or mineral levels. Anxiety or stress can also have an affect. You could be suffering from fatigue or a bad diet.
Are you on any medication?
If none of these help then you will need to visit a doctor to isolate the possible causes.
Take care and please don't worry.