Why is my dogs eye swollen?

Me and boyfriend, live at home with his parents. When we took the dogs out for a walk last night, they all seemed fine. We've been asleep for most of the day (due to shift patterns). I've woken up, and her eye is swollen.

His parents both work office jobs, and one works from home but is in the office today. So the only time the dogs would have gone out today is around 7am. It's now 4.45pm and I'm scared to let them in the garden.

They won't be back from work for another 2 hours. The vet insurance is in their name. So they would have to take her to the vet. I'm not sure what to do.

It's not swollen enough to be an insect sting. She seems okay but is in a lot of pain. Nothing seems to be bleeding or coming out of it. What do I do, please?
Why is my dogs eye swollen?
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