Russian Blue Cat

　　The third eyelid, also known as the nictitating membrane, covers the conjunctiva to protect the cornea, clear foreign objects, and prevent dryness.

　　If third eyelid protrusion occurs on one side, it may be due to eye irritation. Additionally, gastrointestinal discomfort, chronic parasitic disease, loss of appetite, fever, poisoning, or autonomic polyneuropathy may also be causes.

　　Therefore, if this symptom occurs and does not resolve on its own, please take your cat to a veterinary hospital for a veterinarian to identify the real cause. (Source：PetsZone)