A lot of phones have a barometer in them. New iPhones do and most Androids do. Log the data on a 1-minute interval, understand that the data is noisy and the noise comes from moving elevation. Watch the graphs over 1-hour, 6-hours, 12-hour, 2-day timeframes. You'll see rises and peaks and you might note headache timing with a peak or a trough. Migraines happen to a lot of people and can be triggered by sudden changes in pressure. Not sure about your headaches but it's a start.