It sounds like the rattie either got a piece of debris or an eyelash in its eye, or has conjunctivitis (which usually effects more than one eye though). If you can, try cleaning around it with a cotton swab-- you may be able to move the debris even if you can't see it. Use an ophthalmic antibiotic ointment (such as terramycin) to make the eye feel better and prevent infection. You should be able to get that from your vet, or possibly the pet store.