They are caused by fatigue, stress and poor diet. Go to the vitamin section of the grocery store. Look for L-lysine. That is the element that is low in your body. It will not immediately heal them; but will help de -sting and speed up the healing process. Next time you are puting stress on the body take the l-lysine for a few days.
The reason for this is usually an indication that you're body's Ph is 'out-of-balance' and that's usually caused by your food diet. Make sure to drink a lot more water rather than sodas, coffee, fruit juices, etc. Drinking water can allow your body to balance itself naturally.