
Has she recently undergone stress, such as a first season, an illness etc?
It is not uncommon to have a localised for of demodex in young animals that once resolved and the animals immune system continues to mature, does not cause a problem again.
On the other hand some cases get worse where there is a seriously defective immune system or disease.
If it is the localised form and she responds to the treatment, fingers crossed it will not occur again.