Lack of sodium

Add two pinches of salt to a glass of water and drink it.

If it doesn’t go away in about 30 minutes, it’s something else.

Stale air

(Technically: too much CO2)

Tends to happen mostly in the winter.

Open all the windows to create a draft / let clean air inside.

If the headache doesn’t start to improve within 30 minutes, it’s something else.