Swollen optic nerve but normal mri. What does this mean?

Too many to list. Unfortunately there are over a dozen of different possibilities, but an ischemic optic neuropathy, anterior optic neuritis, diabetic papillon is, and retinal vein/arterial occlusions are the most common.
Optic nerve swelling. Lots of things can cause a swollen optic nerve: infection, inflammation, ischemia, vascular occlusion and many other things. See a neuro- ophthalmologist to find out the answer.