I had long had certain headaches that would not respond to my usual headache/sinus remedies, and I would get mildly nauseated if the headache continued more than a couple of days. A dark room seemed to help, but these headaches didn't happen often, and I could usually just ignore the problem.
Then I had a doctor that wanted to experiment with blood pressure medications, and one of the drugs gave me the full tilt boogie migraine, blinding visual distortion, incapacitating pain, nausea, etc.
Dr. Mengele sent me to an ophthalmologist, since it MUST be a vision problem, NOT the NEW DRUGS. When I described the symptoms, the ophthalmologist recognized migraines when she heard them described (she had them herself). She sent him a nasty, rude note about his ignorance and incompetence, I changed doctors, and haven't had a severe migraine since (5.5 years).