about that time, just before your period, your oestrogen levels plummet, which is a known trigger for migraines. I used to get pre-menstrual migraines as a teenager. My first ever migraine that I remember coincided with a period.
Might be worth consulting a homeopath. I'm not sure about HRT either, I think that may have a stabilising effect on migraines for some women during/after the menopause, not sure, be interesting to know.
Women are such a bag of chemicals it's scary :) Like you shouldn't take certain versions of the pill if you have migraines because it increases the risk of a stroke. I believe Microgynon is a notable one, whereas Norgeston/Levonorgestrel is apparently okay.
It's really interesting that people are saying the migraines increase as they've gotten older/towards the menopause. Many women find that migraines ebb off around that time too - although a few put it down to living a healthier lifestyle and being more aware of what they eat and drink than when they were younger.
I would seriously check out some alternative therapies though - homeopathy would be a good starting point but go see a proper advisor rather than just self medicating. Holland & Barrett work on commission ;)