I am sorry your baby is not feeling well. It sounds like she just got something in his eye like, hair, litter, etc.
The third eyelid appears in times of stress, injury or illness.
If the eye was injured and the cornea was scratched, she would need to see her vet.She could lose sight and even the eye without special medication.Corneal abrasians are VERY painful so you want to make sure this in not the case.
If it is just irritated, has a foreign object or conjunctivitis,or only the ouside is swollen you can treat it at home.
Try to get a tube of Terramycin ointment. Some pet supplys and drug stores carry it - but call first or http://www.revivaanimal.com/
In the meantime, apply a damp teabag three times a day.Try to flush with sterile saline for contact lens weares available in drug stores.
If you see any colored mucus (green, yellow) it mens the infection is bacterial and will respond to an antibiotic - I recomend Clavamox for eye issues.
As long as she is eating this is only an emergency if there is a possibiity of an injury to the cornea. If she is acting painful by rubbing her eye I would have her vet check this morning as soon as they open.
Most cats "act sick" by not eating - that is always a sure sign
Please let me know how your baby is feeling and if you still have questions.I will always be here for you
I hope she feels better very soon.
Sincerest best wishes,