Some of the most common factors that may trigger a cold sore include fatigue, stress, fever, a cold or the flu which can all weaken your immune system making you easy prey for a cold sore outbreak.
Cold weather, dry air, winter wind and ultraviolet rays can also lead to cold sores by drying out lips and damaging your skin.
Lastly, the stress associated with hormonal changes and trauma from dental procedures can also cause cold sores.