Don't know if you're still there, OP, but the reason I asked about other triggers is that it was explained to me that most people need a build-up of triggers to cause a migraine: it's very rarely just one thing.
Your main trigger is obviously the onset of your period. Same with me.
So, say you're at Migraine Probability Zero most of the time, but when you're period's due, you go up to Migraine Probability 8.
You need to get to Migraine Probability 10 to get a migraine (this isn't true, obviously; it's just an illustration), so there are probably a couple of little things, each with a Migraine Probability score of 1 or 2 that could push you over the edge.
With me, it's lack of sleep and chocolate.
Once I'd worked this out, I found that if I make sure I'm well rested and avoid all chocolate round about the time of my period, I can avoid getting a migraine every month.
It doesn't always work but it's worked better - and with fewer pills - than anything else I've tried.
Hope that makes some kind of sense.