Allergy Headache vs. Migraine: How to Tell Them Apart
If you get seasonal head pain with facial pressure and a blocked nose, the odds are strongly in favour of migraine rather than an allergy headache — and the two need completely different treatment. Study after study looking at people who self-diagnose "sinus headache" finds that the large majority meet the diagnostic criteria for migraine instead. That is the single most useful fact in this whole topic, because it means the default assumption most people make about their own head pain is probably wrong.
This is not a technicality. If you treat migraine with decongestants and antihistamines for years, you get poor results and conclude that nothing works. If you recognise it as migraine, an entirely different and far more effective set of options opens up.