You ask your vet
It sounds like your guinea pig has mange (mites). You need to take it to the vet because my guinea pig had them before and she started biting her skin, yours will probably do the same. You can buy a decent mite treatment from the vets or sometimes from good pet stores. Do not overdose though and follow the instructions carefully, it is not advised to treat pregnant guinea pigs with this treatment. Pregnant guinea pigs may receive a shot of ivomectin from the vet's office. It is sad that you let it get this far without letting it see a vet. SHAME ON YOU!!!