Hit my head, headache 3 days later?

Not really, you shouldn't have been hit hard enough to do any type of lasting damage. What did happen was that you did traumatized the muscle under the area where you were hit. Take your hand and push on that area for thirty seconds with moderate pressure. In thirty seconds it should be gone. Not knowing for sure where you were hit, the only other thing I can add is if that doesn't take away the pain, follow that part down to your neck and see if when you push in on your neck if it's sore or not. If it is, they you have to take two fingers and push them into your neck as far as you can. Then pinch them together as tight as you can, trapping the cords and tendons in there. If you don't squeeze them tight they will pop out later. After 30 seconds,slowly start to tilt your head to the other side. When tilted all the way, release the pressure and hold your head there for one minute. If you need to, do it again right next to where you just did it to get other tendons that may need help.

Well it depends where you hit it. A light hit to the head on certain places can do that. But it might be a migraine. If the pain is going up and down your neck, then in your head get to the doctor quick! You could loose your eye sight/hearing or both!

I think it's unrelated. So many common things cause headaches and if you felt fine after, it's unrelated.