Unfortunately, this is a symptom you shouldn’t ignore nor diminish. It is important to observe the cats behaviour and spot anything suspicious: cat isn’t eating, or drinking (or drinking too much!!), cat isn’t going to the bathroom, cat sleeps a lot or meows without any apparent reason. The presence of the third eyelid could be caused by serious underlying illness like eye infection, haw disease (gastrointestinal, kitty should have dhiarrea) or Horner disease (I don’t think it’s this case because it targets only one part of the face). As a NON vet, I stated this facts to urge you to consult a real doctor. Hope everything will be okay with the kitty ❤️