So sorry for your DD! Cold sores (fever blisters) are Herpes Simplex Virus 1. It cause sores around the mouth. The HSV 2 is the one that causes sores on the down-there parts. While they tend to stay in their respective places, they can transfer, so it is sooo important she understand this. The virus hides and can stay dormant for years. Too much sun, stress, chocolate, foods high in Arginine can trigger the virus to 'wake up'.
I started getting them when I was 5. My daddy had kissed me when he was 'shedding' which can occur right before a breakout. It is highly highly contagious and your DD probably just drank after someone at school with a blister. Unfortunately, there is no cure and she could possibly have breakouts for the rest of her life. They tend to always go back to the original site of infection. However, she may never have another one...or only get one a year.
I wrote an article years ago about a Japanese study which said an ice cube held for 20 minutes against the tingling spot just as it's starting would reduce the severity.
Lysine, an amino acid, can help. Along with several anti viral creams mentioned. Please make sure she understands the importance of washing her hands if she touches the area, as herpes simplex in the eye is a very common cause of corneal blindness.
Hand washing is a big deal with this.