There are several ways to categorize pain. One way is to separate it into acute pain and chronic pain. Acute pain typically comes on suddenly and has a limited duration. It's frequently caused by damage to tissue such as bone, muscle, or organs, and the onset is often accompanied by anxiety or emotional distress.
Chronic pain lasts longer than acute pain and is generally somewhat resistant to medical treatment. It's usually associated with a long-term illness, such as osteoarthritis. In some cases, such as with fibromyalgia, it's one of the defining characteristic of the disease. Chronic pain can be the result of damaged tissue, but very often is attributable to nerve damage.
Both acute and chronic pain can be debilitating, and both can affect and be affected by a person's state of mind. But the nature of chronic pain -- the fact that it's ongoing and in some cases seems almost constant -- makes the person who has it more susceptible to psychological consequences such as depression and anxiety. At the same time, psychological distress can amplify the pain.
About 70% of people with chronic pain treated with pain medication experience episodes of what's called breakthrough pain. Breakthrough pain refers to flares of pain that occur even when pain medication is being used regularly. Sometimes it can be spontaneous or set off by a seemingly insignificant event such as rolling over in bed. Sometimes, it may be the result of pain medication wearing off before it's time for the next dose.