What Causes Swelling In Optic Nerve Of Eye?

Yesterday I went to my eye doctor thinking I may have pink eye in my right eye (which he said it wasn t). While there he did a brief exam of both eyes and said he could see that my left eye optic nerve is swollen. He sent me home saying to come back in 4 months for a follow-up. I tried to do my own research on this, but it doesn t seem to line up with being Optic Nerve Papilledema since it s only effecting one eye (and he didn t ask me if I was having headaches or any questions). I then read up on Optic Nerve Neuritis thinking maybe that s what he meant, but I haven t had any vision changes in that eye so I m confused as to what the optic nerve swelling may be. Please help clarify this.