What Causes Head Aches On Sitting Hard Or Falling On Buttocks?

When i sit down hard or fall on my butt my head hurts really bad and it hurts for a while. I can also gurantee that everyday i will have a headache. Is this normal? A lot of the people i do martial arts with are surprised by my headaches during training. I stay hydrated so I don't believe its from that. Any ideas? Thanks for your time.