Hello and welcome to the forum.
Some more info and a pic may help - when did you last treat for mites, any history of fungal skin problems, age, how long have you had your pigs, any new arrivals recently? Please list all symptoms, physical and behavioural.
The more info you give the more targeted the advice can be, if you are at all unsure of how to diagnose skin conditions a cavy savvy vet is always better than forum members giving an educated guess. Hairloss from allergies is not common.
If you give your location hopefully there is a good vet that can be recommended to you. Once you have a confirmed diagnosis more relevant advice can be given.
Suzy x