Tension headache. This is the mos common type of headache. It happens when the muscles in your scalp and neck tighten. This causes pain on the sides and back of your head. Usually it's a dull pain that doesn't throb. Poor posture. If you tend to slouch when you sit or stand, that can strain the muscles in the back of your head, upper back, neck, and jaw. It also can put pressure on the nerves in those areas. As a result, poor posture can cause tension headaches and pain in the back of your head.