Although it doesn't say that it suppresses your appetite as a side effect, since Robaxin works on the brain, it is possible that the appetite center in the brain is also affected, thus suppressing your appetite.
You may want to consider trying to eat breakfast or other meals even if you are not that hungry. Set a schedule for eating which you can follow at the moment instead of relying on your body at the moment. It would be bad for your health not to have proper food intake and this may even worsen your migraine attacks.
I can say first hand that whenever I have a migraine and I am hungry, the pain gets worse.

so, I'd better eat especially when I feel a migraine approaching so that I could handle it better. Or else, try limiting your dosage to about 2x a day instead.
Hope this helps.