Under WAC 296-126-050, if you make a written request for a statement explaining the reasons for your discharge, your employer must provide one.
If you were truly fired because you missed work due to migraines, that is disability discrimination. I strongly suggest you contact one of the many employment attorneys listed at www.welaweb.org. (Full disclosure: I am one of those attorneys. But I have too much work, so don't call me.) You likely have a worthwhile lawsuit against your former employer. The disability discrimination should also be brought up in your application for unemployment benefits.