Good Morning and welcome. I am Dr. Loretta, a licensed veterinarian and I am happy to answer your question.
A prolapsed third eyelid in cats can be due to a disease process or may not have any reason at all. If your girl is other wose healthy, active and eating well, there is no reason to take her back to your vet. This would only be required of you see any other symptoms that would indicate a disease process.
Does that make sense to you?
Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please Reply so I can clarify for you
Sincerely, Dr. Loretta