Has anyone developed fibromyalgia after being on prednisone for an extended time?

Another diagnosis? There is no link between fibromyalgia and steroid use or steroid withdrawal. However, prolonged Prednisone use is associated with a condition called 'steroid myopathy' which can mimic fibromyalgia. I'd recommend seeing a neurologist, physiatrist, or rheumatologist to help sort it out.
Unlikely. When a person stops taking Prednisone after having been on it for a long time, it is not uncommon to feel achy and "flu-ey" for a while. The body has to ramp up its own cortisol production and it can take some time for the adrenal glands to wake up.