This could be several things - as with the scratching and sneezing questions [above] are there any other symptoms which could indicate a fungal / mite problem ? Has she recently had babies or is she pregnant ? Hormones can result in bi-lateral hairloss [either side of her flanks]. Ovarian cysts can also cause bi-lateral hairloss. Guinea pigs naturally shed loose hair, as we all do, a little is normal - you will get to know what is normal for your pig - but when the shedding is heavier and is accompanied by other symptoms it is time to have your guinea pig examined by a vet for a proper diagnosis.