Your head weighs about 12 pounds when it sits over your spine. Every inch it drifts forward, from screens, phones, side sleeping, adds roughly 10 pounds of load.
At 3 inches forward, your neck muscles are holding up the equivalent of a 40-pound head all day. They never let go. And as long as those muscles stay locked down, the nerve underneath stays compressed.
That's why the headaches don't just happen on stressful days. They're there in the morning, before you've done anything.