I share your concern for your cat. Is your cat squinting? Is there any discharge in the eye?
Swelling in the area of the 3rd eyelid can be due to a foreign body stuck behind the eyelid, an eyelid mass or infection.
If your cat is squinting or the eye is less open than the other one, you need to have your vet check to see if there is a foreign body stuck behind the eyelid.
I consider eyes to be of an urgent nature. I would highly recommend a visit to your vet tomorrow, before the weekend. It is likely not too serious, but even if it is an infection, it can become serious if left untreated.
Please let me know if you need more information.
OK, since there is no discharge that's a good sign. I'm glad you'll take her to get checked. Eyes are not something I ever tell clients to wait on. It can wait until morning, but definitely have it checked before the weekend.
An office call is around $35 + about $15 for meds.