Some sodas have alot of phosphoric acid in them (mostly cola types).

Also diet sodas with aspartame, contain aspartic acid.
This amino acid triggers the pain receptors called NMDA receptors. (magnesium blocks this action...so if you are low in magnesium like 70% of the rest of the US, then you may have more pain).

Some people get headaches from aspartame.

When people have a chronic idiopathic problem, it is best to examine the diet and foods/water consumed. I, for example, am triggered by nightshade veggies, and MSG. I can tolerate fresh tomatoes if I scrape out the seed pulp, but potatoes seem to be much worse for me. Nightshades contain solanine, which accumulates in the body and may rise up to give symptoms in some people. Others find gluten is a culprit.

I drink diet teas (Snapple and Lipton green tea), once in a while I have a diet ginger ale (both have aspartame mixed with other sweeteners). Some aspartame doesn't bother me or my husband. But I know some people who get headaches from it. I use Splenda in my homemade tea, and on oatmeal.

When you suspect a dietary trigger, it would be best to eliminate it for a month, monitor your symptoms, then reintroduce it. Do only one thing at a time..to see what happens.