Just like a physical map, or the map on your phone, your brain also contains a map. 

So what is a map actually? A simple definition is that a map is a a diagrammatic representation of an area. It's a symbolic interpretation of a place and highlights the relationships between elements in space, either perceived or actual.

Every part of the body is reflected in a specific part of the brain known as the Primary Somatosensory Cortex. The map is know as the Homunculus. This is basically a representation of the physical body located within the brain. It is a neurological "map" of the anatomical divisions of the body that receive sensory input from your entire body. It serves as a proportional system of processing touch.

So when you feel heat, or touch, or pain anywhere in your body, the information is sent through the spinal cord up to the brain, and on to the sensory cortex.  Information from your fingers would go to one area, and information from your shoulder would go to another area. So, If the sole of your left foot is brushed with a feather, an area of the sensory cortex becomes active. If you have pain in your neck, a different area would light up. 

On the Homunculus, areas that are larger indicate the most sensitive areas in our bodies. The hands, the tongue, the genitals, and the facial features are extremely important, and give us a lot of sensory information. As a result, they take up more brain space.

Brain MRI's of patients with chronic back pain have shown an increase in the area of the sensory cortex associated with the back. When you suffer from pain, that virtual representation of the affected limb can become significantly altered. As your brain begins to pay more attention to the affected area, it can become more sensitive and enhance your pain. 

Other changes in the somatasensory cortex can lead to more diffuse pain, or when you are unable to specifically locate your pain. This may also be why someones pain may initially start in one area such as the knee but then begins to effect the hip, as greater areas of the primary somatosensory cortex are activated.