Pressure, empty numb, tension feeling in back of head. Laying on it or wearing a hat makes it worse. What can this be? No pain associated. Serious?

Headache. Tenderness could indicate a muscle tension type headache. Rarely, migraines can have a muscle spasm component associated to it, specially in the neck area. Also, neck conditions can manifest with headaches. Check with your physician for proper diagnosis and treatment. Hope you feel better soon.