As I've said, chocolate is a major trigger for migraines. However, if you view the link that I provided, you'll see that there can be many other possible triggers too. Obviously not every single person will get a migraine from every single trigger or every single time. I believe the link provides a list of the most common triggers.
Tension headaches are not the same as migraines. However, I wonder if the original poster truly knows what causes her headaches. There are migraine headaches, tension headaches, withdrawal headaches, allergy headaches, etc. Even though the doctor may call it a "tension headache," it's entirely possible it may be caused by something else (grinding teeth at night?), as some of the posters have suggested.
The soda is bad for you whether or not you have IBS. Outside of the IBS issue, the soda is full of sugar which is not good for anyone. Too much consumption of foods with added sugar can lead to overweight and, even worse, diabetes.
Or, if you're drinking sugar-free soda, it's full of awful chemicals used in artificial sweeteners. Some of those chemicals are cancer-causing! In my mind, sugar-free soda may be worse for your health than sugar-added soda!
One of our Canadian school teachers likes to demonstrate to her students first-hand how bad some sodas are. So she brings in a bottle of cola and drops a penny into the liquid. She then reseals the bottle and the kids watch over the ensuing weeks as the penny completely vanishes -- literally disintegrating in the soda! Yuck! I don't care how good cola tastes, I wouldn't want to drink it!
I still think what you are experiencing may be a caffeine-withdrawal headache. However, since you are prone to migraines, your withdrawal from the caffeine soda may be causing you a double-whammy -- a withdrawal headache and a migraine. Ouch!
The good thing is once you overcome that caffeine dependence, the headache should dissipate. By the way, are you getting any shakes (trembling hands, for example) as well? That's another common symptom of caffeine withdrawal.
I'm sorry if I sounded rude in my previous posting. My posting wasn't meant that way. I tend to be very direct in my messages so perhaps I am not always as diplomatic as I should be. It's just my writing style.
I hope you get off that soda very soon! You'll feel a lot better when you do. Try replacing it with a room temperature, non-carbonated bottled water (Evian, if you really want to splurge!) and cups of peppermint tea, fennel tea and camomile tea (the camomile only if you don't have any Hayfever/ragweed allergies).