Headaches 2-3 days after XC marathons

I will also go to the doctor after my next marathon and get some blood work done. But I'm interested in other people's experiences.

Last year I started joining a few XC marathons in the area where I live. (I didn't push it too hard since they were my first marathons) This year I had 3 and one more incoming and I'm pushing it at 90-95%. Average BPM is usually around 160.

I noticed a pattern, a couple of hours after each marathon is done I get a head ache that lasts 2 maybe 3 days. I tried a couple of things, drinking gatorade, oral rehydration salts, eating what should in theory contain Sodium but it still does not work and sometimes it's hard to sleep the first night with the head ache.

Do you guys experience the same issues? What do you do? Have any advice?