Yes it is quite normal to experience diarrhea before during or after migraine, it is classed as an autonomic symptom as is nausea and vomiting.
Hope this helps, take care
Co-Moderator of the Migraine/Headache Forum
DX: Hemicrania Continua, Chronic Migraine, IBS, Allergic Rhinitis and Heart Murmur
Meds: Imigran Injections, High Flow Oxygen, Mebeverine, Lansoprazole, Nasonex and Clarityn