They are caused by sun and stress. Based on your story sun is more likely your culprit. Take L-Lysine daily and apply SPF lip balm before you go out in the sun and reapply on long rides (try 'softlips' brand). One yogurt for breakfast each day is also helpful. Your MD could also prescribe Valacyclovir to take once you feel a cold sore coming on. ^ StephenH is correct - they appear about three-days post (sun) trauma.