When my cat was 17 she developed a runny eye which we initially tried to treat with vet prescribed eye drops. A few days later she stopped eating and the vet re-examined her, sedating her to check back teeth. The vet found a tumour which was growing down from the sinus area. I hope your cat will be okay and the vet finds something treatable.
Please have pets spayed and neutered,
and wearing a collar with an ID tag.