To the person with the cat who had the string hanging out of it's butt...sounds like the cat ate some string which made him/her sick which caused the third eyelids to show. If the string is no longer visible but the cat still looks sick, take him/her to the vet. There may be some other piece of material still inside the intestines causing a problem and it could be serious.
When my cat ate about a foot and 1/2 of dental floss the vet said to bring her in. They tried to induce vomitting but it didn't work. They said to give her that hairball remedy goo (Petromalt, Laxatone, etc.) until the floss passed. It worked and the floss passed out of her all balled-up in about a day and 1/2. Easy as pie. They said it could have tangled up with the intestinal contents and become serious and required surgery, so it was quite a relief to see it come out all at once.
Sounds like your cat ate something that isn't passing smoothly and it could be life threatening. I would take him/her to a vet NOW.