This page explains the difference.

http://www.rsdhope.org/crps-or-fibromyalgia.html

I have both, and they do feel different to me. I believe "they" say that most people with RSD develop Fibro. A couple years ago, I had RSD spread to my upper back. I have temperature, color changes and sensitivity in addition to pain. I asked the doctor I was going to for RSD about it and the doctor said RSD does not spread, that my back is caused by my Fibromyalgia getting worse. This is the same doctor that has diagnosed me with RSD in right foot, then a couple years later in left arm and shoulder. So, then I went to my rheumatologist, who is an expert in Fibro, and she said has never seen the color or temperature change in Fibromyalgia.

So, I stopped going to that pain management doctor.

The neurologist I go to, who originally diagnosed the RSD years ago, thinks RSD can become Central Pain Syndrome, which he thinks accounts for the spread.

All the doctors seem to have different ideas and opinions.