Eye injuries and infection(pink eye) can cause this to happen. If there's abrasion on the eyes the third eyelid will come up to protect the eye and the dog will squint heavily and appear painful. You may need to have the dye put in the eye to check for ulcers. Some ulcers will heal on their own and others need an eye specialist that can do a quick procedure called a corneal grid. Worms can cause eye issues too, so please rule out parasites at your vet.

If it is something new and you are worried, see a veterinarian with her. On the other hand, it just may be her. Plenty of Labs sleep with their eyes partly open, and the third eyelid does a nice job as it keeps the eyeball from drying out while they sleep. It just startles the guests when they see this seemingly blank and ghoulish stare, sometimes bright pink, sometimes white, on an unmoving dog! I have seen it in both show bred and field bred Labs at home and in rescue over the decades.

You are not talking about a persistent red bump in the eye corner as in cherry eye, are you? That needs a vet and at minimum medication.

Not cherry eye but as you described looks like she sleeps with her eye open that's when the third eye lid is present. I'll have a stool sample checked since its been a year since last checked & these guys love to find other species poo in the woods.