If your work performance really is suffering, your employer can give you poor reviews. You might want to take this opportunity to talk with someone in employee relations about FMLA.
Who is treating you for Migraines. You may want to check out an actual Migraine specialist. You can find a listing of patient recommended specialists HERE.
Employee rights can also very from state. It couldn't hurt to call the closest Department of Labor office.
Good luck,
Teri