Sympathies to talyco and all the other sufferers. It's rotten isn't it? I get immobilising sinus headaches once a month, regular as clockwork, usually just after af but also if I have been unusually stressed or dehydrated. At that time, I wasn't sure what was causing the pain and it got so bad that I went to see my optician, gynaecologist, gp and dentist. Here's what they said (if it's any help):
optician - all OK - eyes not implicated
gynaecologist - couldn't help but said that the same hormones that 'stop the flow' after your af can have the same effect on other secretions in the body (sorry if too much info!) thus bunging everything up
dentist - couldn't help but remarked on 'my sinuses being the size of bellows!!'
gp - again couldn't help but blood test came back as me being very allergic to dust, or more specifically, dust mites.
Even though the regularity of the headaches must mean that there is hormonal involvement somewhere in all of this - I do now tend to always sleep with an open window - dust our bedroom v. regularly, change sheets very regularly and generally keep environment as dust-free as possible (not easy because we're renovating). Also, however tempting, I never lie in bed even when suffering from an excruciating headache. Much better to suffer upright!!
P.S. My brother (same bellow-like sinuses) had them flushed and the operation was painful, expensive, and totally ineffective ....