Fell on my stomach and hit my chin on ground, have slight pain in back of head, otherwise feel fine, could there be bad brain injury?

Pssible but unlikely: Your chin probably absorbed the force of the blow. Still, in rare cases their could be a mild concussion or other injury. If you have a headaches that won't get better, vomiting, blurry vision, or are sounding drunk (slurring) to your friends or family members get to an emergency room immediately . As long as it gets better over the next few days you are probably ok

Cervical spine : You should see a physician to evaluate and clear you of cervical spine injury, tetanus vaccine update if you broke the skin and have not been boosted, and to evaluate you for jaw, dental, skull and tbi.