What makes migraines start?

Look for trigger. Although we don't know the exact causes for migraine, we can look for trigger setting of a migraine: sleep deprivation, stress, chocolate, red wine, msg, weather changes, cheeses, hunger and many more.
Blood flow changes? The actual cause of migraine headaches is not known. The vascular theory says that changes in the size of blood vessels leading to the brain causes changes in blood flow, with increased flow causing pain. The neurovascular theory says changes in brain signals and brain activity cause blood vessels to change sizes, resulting in pain from increases in blood flow. Additional theories also exist.