Why do I get headaches 12 hours after exercise?

Exertional headache? Exertional headaches are a primary headache type. They are brought on by physical activity. They last 5 min to 48 hrs. The trick is to either avoid the offending activity or take chronic medication to help prevent them. You can also try to scale back your activity and slowly increase to increase your tolerance. Dehydration can also do this. Definitely worth seeing your doc about this.