why are my cats third eyelids halfway showing

This is my cat sap, him and his brother (from the same litter) are indoor/outdoor cats at the moment. Yesterday afternoon I got home from work to grab my stuff and make sure my cats had enough food and water and then leave to my boyfriends house. And when I was about to leave sap cam up to me for a little and I noticed his eyes were like this but I didn’t really think anything of it just maybe he was tired. So I left and when I came back home at around 6pm today he was awake and greeted me normally while I was walking to my front door for both of us to go inside. He was wide awake and alert he has been meowing louder than normal he usually has a very small high chirpy meow but its been loud and whiny sounding since yesterday and it is not 8:30 pm and his eyes have not stopped looking like this. He has never had anything wrong with his eyes or third eyelids. Please help if you have any advice or knowledge for whatever this may be!

Hello - having slightly closed third eyelids is indeed a sign that all may not be well, but it is a very inspecific one that can have many causes.   Trauma and damage is one possibity - things like dust / allergy / grass seeds etc that might affect both eyes together should be considered.  Alternatively, it can be a sign of damage or pressure to the nerve holding the third eyelid open, so can occur in upper respiratory tract infections and viruses for example, or lumps and bumps impinging on the nerve.  Endocrine changes and abnormalities of the eye itself such as pressure changes or tiny scratches could also be considered.  Im interested that the meow has changed because this implies a change to the tightness of the throat and makes me wonder specifically about changes to the nerve supplying that area.  For this reason, we would reccommend speaking to your vet as soon as possible about this change.   Best of luck.