It can either be 'barbering' -either your pig or her cagemate is nibbling the hair
or
if the hair looks 'snapped off' it is a symptom of mites.
Best plan is to treat for mites (catch them early if you can)
If it doesn't respond it could be fungal. You'll need different treatment for that.
Gorgeous Guineas have a range of shampoos, melts and souffles. Have a look on their website. They should be okay to bath if they are in.
I use Xeno 450 (just ordered a new box for my boys. Too cold to bath them, mine are outside)
If it is barbering ,is it boredom? They might not have enough to occupy them indoors?
Scurvy can causes hair loss- are they getting enough Vit C?
Over heating making the skin/fur dry?
Do you have them on shavings? (Dries the fur as well as being bad for their respiratory system)
The hay is dire at the moment, dry, brittle (hay can be a carrier of mites).
Good Luck. If you don't see an improvement, see you vet. If it is mites, it can kill.