This can be caused by a variety of micro-organisms, but commonly one of the 'flu viruses is responsible. Both feline calcivirus and feline rhinotracheitis virus will produce stomatitis but feline calcivirus does so more commonly. You need to take your cat to the vet it may need surgical therapy if it can't eat. Make sure that your cat drinks water so it won't dehydrant. If it won't drink take it to the vet it may need an IV. Good luck!