It's pretty impossible to make a diagnosis. But if only one eye is affected, the first thing I would consider is injury. So as part of my exam I would use an eye stain, then using a special light, look for corneal ulcers and abrasions. I'd also do a physical exam of the eye looking for foreign bodies. I once found a blade of grass buried in the eye folds beyond what is normally visible. Another time I found a small bead that apparently a young child had poked in there.
Eye injuries can be the result of all sorts of things. Riding in a car with the head out the window. "Helping out" when owners are mowing and weedwacking. Running through brush and overgrown grass. Running into objects. Falling. Rubbing against an irritant (laundry
soap containers, bleach bottles, rubbing down the length of the sofa). Getting swatted by a cat. Rough housing with other dogs or animals. Playing with children. Getting bitten by insects and other "stinging things".
Another thing to consider is an inflammatory response to something. Getting something accidentally into the eye -- soap, flea spray, spray paint mist, etc. I had a client who's Bassett Hound suddenly had an inflamed eye that we couldn't seem to correct. Through much questioning it was learned that the owner had purchased a new plush dog bed which coincided timewise with the one eye problem. Observation revealed that the dog always slept on one side, bad eye down. Washing the dog bed several times in a row resolved the eye problem, which then never reoccurred.
Infection is always a possibility but it is not nearly as common as injury or inflammatory response.
I'd suggest a trip to your vet. Best to make sure it's not the something that could progress into major ulceration and thus loss of the eye.