Okay, you're more informed than I am, I haven't worn a mask in my life (the closest I got is having to wear a bandana while biking because it was -30C).I’ve worn a surgical mask for several hours, on multiple occasions. As a med student and as a junior doctor. Never quite a “full day” but often up 4 to 6 hours at a time. I never once got a headache from it. Nor did any of my peers. I’ve also never heard of this as an issue from surgeons, anaesthetists, theatre nurses and porters, who wear surgical masks for the majority of their working lives.
I do know that the N95 respirator masks can be quite uncomfortable. And I wouldn’t be surprised if they cause a headache for some people that wear them. They definitely cause skin irritation and rashes. They clamp tightly onto your face and form an airtight seal. Which is not nice when worn for hours at a time. That’s not what the general public are being asked to wear though. In fact they’re being encouraged not to wear this type of mask.
I suspect @RobinLFC’s aunt is wearing one of these masks, hence her bad experience. Because, as I said, the idea that the light cotton face coverings/disposable surgical masks the general public are supposed to wear might cause headaches really makes no sense at all. And if someone has lived in a country where wearing face coverings like these has been a common occurrence for many years and says that headaches are not an issue, then we can safely assume they know what they’re talking about.