Why is the back (crown )of my head sore?

Many possibilities: There are many possible causes. A couple key questions are did it come on suddenly and is it severe? If so I would be concerned about a possible small bleed into your brain and you should see a doctor soon. Otherwise do you have any changes in your scalp? Is your neck sore? Do you have a fever? It would be a good idea to have an online consultation to try to clarify what is happening for you.

Headache or Trauma: It's not possible to know without having more details about the soreness; however, I can say that a very common cause of head pain is a headache. Tension headaches are often felt in the back area of the head. Soreness could also be due to trauma to the area; this might be the case if you bumped your head. There are other possible causes, as well.