Chronic pain can develop after a major injury or illness, such as a back injury
shingles, or it can happen without a known cause. It
is also possible that certain brain chemicals that usually suppress pain stop working the way they're supposed to.
Nerves and the nervous system. This type of pain happens because of pressure on nerves or damage to them from an injury or a health problem. Sometimes pain occurs when something goes wrong with the central nervous system. This can happen with diabetes, shingles, and sciatica, for example.