I was diagnosed with fibromyalgia a while back. I'm not 100% confident in that diagnosis, but I want to share my experience because it might help others.
At the time, I was working 9-5 at a computer for the first time in my life. I was also rowing, and often didn't stretch as much as I should have. But I think what triggered my fibromyalgia was my leisure time, during which I spent long hours on the web at home. The reason this became so detrimental is that I felt very guilty about it afterwards. And when I was feeling guilty, I started paying too much attention to my body, always checking it for pain, afraid I was hurting myself, wasting my energy. I'm pretty sure when a healthy person asks themselves how their body feels, very often they will find some pain. The problem, in my case, was that I let that pain grow in my head, and I let it tell me that I had to rest more, take more medication, and that I wasn't up for exercise. Once you start accepting that you will feel a little pain after exercise, work, and other activities, you can start creating positive associations with the pain in your head. People who work out feel less pain, but also enjoy the pain they do feel, because it's an accomplishment for them. Once I stopped paying so much attention to my pain, this condition of increasing pain, fibromyalgia or not, went away.
I know some people will find this post stupid or misleading. This is just my story. I think it could have been similar if instead of guilt, my pain trigerred anxiety, depression or self-pity. This is my opinion, and I put it here because I truly believe it might help others.