Definitely mild hypoxia
On long haul flights especially at night and occasionally on short haul night flights we used to don our masks before TOD, within a minute or so vision improves markedly and that's with a max cabin altitude of 8,000ft.
I used to fly in the alps and limit time above 11,000 ft to a couple of hours and rarely climb above 13,000 but it didn't stop me from getting a massive headache.
The longer the time the longer the headache persisted.