I experienced those headaches early on… really intense pounders that would essentially force me to stop the workout, and they would last for hours and even days. The first one I got, I considered going to the ER. At the advice of my coach, I made the following changes:
I increased my sleep overall, especially the nights before a WOD. I would try and get a minimum of 9 hours sleep. I hydrated like crazy, even drinking a pedialyte or gatorade on the way to the box. And I made sure I had a good breakfast a couple hours before, lots of protein, fats, and good carbos. The headaches stopped immediately, and over time I was able to dial back some of these measures and determine what, for me, was the thing I needed to do to avoid the exertion headaches… for me I found I don’t need QUITE that much sleep, I don’t need the pedialyte at all, but I do need a decent amount of food. My breakfasts are some sort of meat, almonds mixed in plain full fat greek yogurt, an apple, and a lot of water.
As far as your neck, I don’t know. I assume you have a good coach so just ask them, it may just be a matter of a minor correction in form or the need to stretch better. If your gym offers a mobility class, try it out (it was hugely helpful for me). Yoga is good too, for a variety of reasons. If i had the time in my schedule, I would probably do 50/50 yoga/CrossFit.
I was never particularly athletic… I ran track in high school and was a skateboarder and snowboarder for a good while, but I dislike formal competition and in no way do I have the mentality that most people think of when they think of an “athlete”. Which is why CrossFit, I think, has been so great for me. I like the data-collecting aspect, the ability to scale and adjust for a personal fit, to compete against myself, and the range of people and types that I share my workouts with (all ages, all genders, all body types, etc).
Good luck, keep me updated.