When a migraine happens - there are a host of changes across the entire body. Migraines cause a "spreading cortical depression," which is just a way of saying that a wave of lowered activity spreads across a certain area of your brain. When it reboots, nerves go haywire, blood vessels dilate in the brain. For some people, the digestion halts to a slow or stop - called gastroparesis. This is what is to blame for the cyclical vomiting people will have.
Some people will notice a day or two before their migraine hits that they are experiencing mood changes, or urinating more. This is a sign of that cortical depression in effect.