Very sorry to hear about this problem.
Even in the absence of discharge or eye inflammation, we cannot rule-out an injury to the eye. If one eye also appears larger, we cannot rule-out glaucoma, where there is an increased pressure inside of the eye, which can also be quite painful.
Even at this age, glaucoma is still a consideration.
Without question, I would strongly recommend examination by your local veterinarian, and ideally go today.
If there is a concern for glaucoma, they can do a easy eye pressure test called tonometry. If there is any concern for a injury to the surface of the cornea, a eye stain test can easily detect it.
Unfortunately, we have no way to appropriate treat this concern at home, as well as, no idea if it is going to get progressively worse quickly.
Well worth the visit to tackle the eye pressure concern, and figure out if there is a quick topical the vet can prescribe to help the situation.
Yes, I would have to consider a incident at the groomer as one possible explanation.
I hope that information helps