Here's the thing, though. The headaches for which we are certain of the causation for are the headaches in Small Favor, where Mab hides the information from Harry about his blasting rod. So let's start there...
The description from those two headaches in SmF. For the first headache, "ice picks plunging into both temples," and for the second, "savage pain." So even though they both are caused by the same thing, Mab, the description is not the same. Arguably, a third headache caused by Mab (Ms. Duck theory) occurs in TC where Harry is precluded from realizing Mouse could help him find Thomas. This headache is described as a vise on Harry's temples. So even with the same agent causing the headaches, the descriptions vary.
That being said, 2 headaches occur in SK and GP which both are described as at the base of his neck creeping towards his crown. Each occurs while Harry is researching with Bob (looking up info about the Queens and werewolves.)
FOUR headaches occur all described as being "between Harry's eyebrows"- three in WN and one in Changes.
The only other echoed headache is the one in Cold Days, caused by the parasite, but described as "two separate ice picks in his temples", "in the same place as usual" indicating other headaches occur here. But the ice pick headaches clearly correlate to the Mab geas headache in SmF. [tangent - Is this why people are saying maybe Mab made a deal with the parasite to block something else from Harry?]
Finally, Harry thinks about a several month time period just before TC where he started having and continues to have increasingly horrible migraines.
So I don't know how one looks at this collection of headaches and reaches the conclusion that Lash is causing specific headaches at a specific time. It seems like there may be a pattern to certain headaches or connections between them, but its thin at best. It seems to me that there is more evidence that something funky is going on before Lash dies, with the "between the eyebrows" headaches than afterwards, unless the lumped together "increasing series of migraines" are the headaches everyone attributes to Lash's regeneration.