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.