Hi
Welcome to the MedHelp forum!
You have probably jarred the nerves at the back. In all probability this will settle down with rest, pain killers and a hot water bag. Apart from this watch out for nausea, vomiting, excess sleep, severe headache, difficulty in walking and balance, tremors, seizures, vision problems, and ringing in ears. These symptoms develop if there is a brain injury or bleed. A MRI will be required then.
Since neck and back of the head is a sensitive area please consult your PCP for a primary examination.Take care!
I would go to the ER and have a scan done. There could be some slow swelling and/or bleeding going on. Could just be a headache, etc. because you hit your head but I would get it checked to rule out more serious possibilities.