That is the nictitating membrane (third eyelid) swelling. It usually is because the animal is stressed. They can do it at will. If not stress, it might be an irritation. Try flushing it with saline solution, then put in a dab of antibiotic ointment. And then again, it might be due to an infection someplace else in the body (similar to an ear abscess). I had a gulf coast box turtle here that would do that every time I picked him up. I sent him to Danny and he says it doesn't happen anymore. I guess he likes Danny better than me.
I have an Eastern box turtle who does that too. In her case I'm pretty sure it's stress. I would put a dab of Terramycin antibiotic ointment in both eyes twice a day for a few days and see if it changes. What could be happening in her life that makes her feel stressed? It also could be an infection like Yvonne says...
Um well she did just get a new house but it was there before that. Besides that she came in from outside in sept. She has been gaining weight steadily still and has not changed any of her habits bath in the morning then some sun and I keep handeling to a minimum. Her frien that is with her is fine and they do not seem to be bugging each other at all. Maybe her friend scratched it?? I will flush it and see how that goes though I did that once already and nothing changed I will try again