I would also suggest the elimination. It seems to be the most likely culprit, and sensativity is more common than the manufacturers want you to think. If you are or become sensitive to artificial sweeteners a even little bit can bother you. I have had sensitivity to aspartame for a long time. Even too many pieces of sugar free gum will give me a headache. I was able to use Splenda (sucrulose) for a year or so then it started bothering me.
It is really a huge pain when trying to lose weight and still enjoy stuff. I can't have any diet drinks, powders ect. Yogurt that is calorie friendly and doesn't contain artificial sweeteners in it is impossible to find. Some people can't even use certain brands of toothpaste or mouthwash b/c they contain art. sweet.! Luckily I am ok there. Plus, more and more products like cereal, flavored water, juices, pasta sauces contain artificial sweeteners with out showing the big Splenda or Nutrasweet label on the front of the package. I have to carefully scan the ingredients.
There are alternative sweeteners like stevia, which is highly used in Europe and Asia. It's touted as all natural, but then again so is arsenic
It has been linked to some fertility issues in men. I don't know a whole lot about it but I choose not to use it b/c of teh fertility thing (even though I am a woman).
Best of luck I hope you find out what it is. I have been there and daily headaches are awful!