Is she acting normal otherwise? Its better to go to the vet, but for a little piece of mind, I don't get too worried unless they are lethargic and not drinking normally. That's when I go the the ER. If she's acting normal, I'd wipe her eye gently with a warm wash cloth and wait till morning. Call your regular vet and lean on them to work you in. They don't stay in business very long turning regular customers away.