Why do I get a severe headache just before landing?
Your sinus passages were obstructed and it was atmospheric pressure squeezing your skull as the increasing pressure on the descent tried to equalize the pressure.

The air in your sinuses escapes much easier than it goes back in so the problem is always after a descent.

Feels like someone trying to drive a nail into your skull between your eyes.   Plug your nose with your fingers with your mouth closed and blow GENTLY and you will feel the air enter your sinuses and relieve the pressure.

Good idea to take a sinus medication like Otrivin with your travel kit if you are having that problem regularly.