Maddie had this same problem. It started as what was believed by one vet to be simple conjunctivitus. After a tube of ointment didn't work, I took her to another vet. There she was diagnosed with dry-eye. It just happened all of a sudden.
Her tear duct wasn't working properly, so her eye was creating mucus to provide moisture. And it did stink!!!
Now simple over the counter eye drops applied a few times a day keep her eyes looking fine. Good Luck