What might be the cause of a temporary smudge in the center of vision in one eye followed by days of pain behind the eye?

Central blurred: vision that is transient could be a vitreous body opacity (vitreous floater). That in itself should be evaluated by a dilated retinal exam for possible retinal tears, breaks or even vitreous hemorrhage. The pain behind the eye may or may not be associated with the visual disturbance. If it is related, this could indicate an optic nerve issue and you should be evaluated by your Ophthalmologist ASAP