I've wondered too--about pain issues & my first Dx : FMS . after I was hit by car, w/ TBI & other injuries. Like you I didn't respond well --couldn't tolerate or got allergic reactions to most of the meds tried (antidepresants, AED's, & others) . PT usally made me feel worse, except for warm water therapy which I'm trying again to access. Sleep problems continue despite many meds.... resulting in Fatigue, which can be my biggest complaint .. until I get those more intense flares & zaps of pain, which nothing seems to relieve....
anyway, 10 years later,
although many aspects of FMS seem very applicable, felt there was something else, or more going on. IF another or different Dx would enable/allow better more appropriate interventions & treatment, then I needed to find ... so in past few years, more DRs/specialists, & more meds & interventions -most disappointing but some meds have helped.

in process of trying to learn more, educate self, find ways to cope w/ what felt like a life long battle ... I stumbled on description of CPS and support group who seem to know more about it (and Pain issues) than most Drs. The group felt I met the criteria for CPS but no formal Dx.
Most of them developed Central Pain after having thalamic strokes but other trauma to central nervous system can cause CPS including toxins, TBI, spinal cord injury also MS & other conditions that affect Central Nervous System, and mess w/ those parts of the brain that TJean described in her post, resulting in central pain sensitization.

some believe this also occurs w/ FMs. the problem is in your head, not in the fibers & other connectors as the name implies. One Dr believes hypocampus is affected & shrinks (per his research w/ FMs patients using PET or SPECT scans saw [things in] Brain other technology didn't pick up]

others may be able to explain better as well as have different view but what I've learned is similar to what TJean shared-- RE: central pain sensitization--in both CPS & FMS, but what causes this may differ.

for me, the cause of my pain was traumatic injury to my brain as well as other body parts, from accident. I feel that Central Nervous system is the big player in my pain, but also other systems are affected, like peripheral . I have multiple med issues, most involve Pain, some w/ DX , that may be accurate or may not, be totally inclusive.
There is overlap and many disorders can/do occur concurrently. we may not have ALL or most appropriate Dx that will enable better or Best treatment available. We need more... research, understanding, education.

check out the sites/Links in prior posts for about CPS.

Hope we'll ALL get what's needed soon...
.