It could be an injury, or it could be a fungal infection, only a vet could properly diagnose. If it's fungal, your other piggy might show symptoms soon. If you are in the UK try the PDSA, they are vets for people on a low income, and may be able to help. Don't treat it yourself, as it may disguise symptoms and make it harder for a vet to treat. Be very careful handling your piggy, wash hands before and after, as some fungal infections can transfer to humans - best be on the safe side. Good luck.