If you don't mind me asking, are you male or female? If female it could be an early pregnancy symptom. However if you google it, you will find that it could be due to a change in ones diet, anti-biotics, oral infections... the list continues. Some sources report that it is due to a lack of vitamins/minerals in the body (vitamin B and zinc). Also a high protein diet could be the cause.
Try taking a general vitamin everyday and see if it helps you out. You could also talk to a nutrionist to make sure you are getting the recommended amount of vitamins in your meals.
what kind of diet are you on? If it's that all meat diet, you could be experiencing Ketosis.