I have a tender spot at the back of my head. It's sore to touch and it feels like a hard lump. Like a bone. What could this be?

May be cellulitis. May be cellulitis or an early abscess if it is tender. That is when it can cause pain.
Scalp Boil. Boils can be painful but easy to treat. Typically, boils resolve on their own, but if a boil gets very painful or gets worse, you should see doctor.