Prolonged stress - chronic pain in particular, can also deplete/suppress cortisol in the long term. Your adrenal gland keeps pumping it until it's... well, pooped. Some sources call this adrenal fatigue (or exhaustion), but much of the medical community doesn't recognize this.
http://en.wikipedia.org/wiki/Adrenal_fatigue
What they will recognize is abnormal levels, whether high or low.
A first step might be to have your adrenal hormone levels tested (simple blood draw w/ 8 hr. fast) to find out exactly what, if anything you're dealing with - which hormones, and whether they're elevated or depressed.
Doc