Advil causing migraines?

Hi Everyone!
I've suffered from migraines since I started college. At the beginning of this year, I decided to change my eating and exercise habits. I became vegan, upped my fruits and veggies, and worked on my binge eating issues, as well as upped my exercise a little (though not as much as I would like - still working on that).

For the most part, since then, I have gone from having 2-3 day migraines every 2-3 weeks to having almost none. I did have one migraine for a day and a half a few weeks ago, mainly because of TOM (which I expected). But other than that, I've been very pleased that my new habits have cut out my migraines.

However, I've been having a really weird thing going on in the last few days. The weather here in my part of Texas has been very up and down (in the 70's for a few days, then way down to 30's or 40's, then back up to 70's). This has caused me to have some sinus headache pain (though not migraines).

The pattern in the last two days has been like this:
I developed the sinus headache during the morning. At about 2 or 3 p.m., I took two Advil gel caps. That made the sinus headache go away for about half an hour and then suddenly the headache is back and starts to turn into a migraine. By about 5, I am in migraine mode. Then, I go to take a nap for a few hours and when I wake up, no more migraine.

It's really weird because my migraine pattern has usually been that if I develop a migraine during the day, it hangs around the rest of the day and I go to sleep with it. It might go away by morning but usually it doesn't.

So it makes me think that maybe it's the Advil that is actually causing me to have a rebound migraine and the migraine goes away once the Advil wears off.

Obviously, I'm going to stay away from the Advil when I have the sinus headache (since it's not excruciating pain that is debilitating - it's just annoying). I'd rather have that than the migraine. But it's just really odd that this is happening.

Thoughts?

Tam