Your dog may have a condition where the eyelids turn inward allowing the eyelashes to cause damage to the eye. It's extremely painful for the dog and makes additional infection likely. In short, since you see no obvious reason for the condition, it's time to have your vet take a look. This condition isn't something you can treat yourself. :-)
My suggestion is to take her to the vet. It can be almost anything, but if they are constantly watery and red, she's suffering and needs your help.
Good luck. Let us know what the vet says.