As Dr. Hagen says, you can certainly call and ask for an earlier appointment because of your severe pain.
Just to add to your information bank, though (since your opthalmologist did suggest migraine), you might want to use google or wikipedia to look up an extremely painful, behind-the-eye kind of headache called 'cluster headache' and see if it describes what you're experiencing.
I have had pain and pressure behind my left eye as well as Optic Neuritis. I have found that my sinuses have lead to increased exacerbations of my MS. I use a NettyPot, now. It has helped a ton! I used to have to be on very strong antibiotics,(Levaquin) every other month for years and now I am proud to say that I have been antibiotic free for about 2 years..and I'm feeling great! The Netty Pot takes some time getting use to...but it is well worth it. Keeping my sinus cavities clear has made a remarkable difference in the number of MS exacerbations I experience. I have been diagnosed with relapsing-remitting MS since 1985 and was told I have secondary-progressive MS in 1998, but those are just labels! We are not labels and I refuse to give up! Yoga helps a ton, too!
No one can tell you what it is over the internet. It could be optic neuritis. You can call the neuroophthalmologist ask for an appointment sooner and/or ask to be called if someone cancels an appointment before yours is scheduled. It could be other problems like orbital inflammatory disease.
JCH MD