My two cents:
Prednisone can absolutely cause or increase muscle pain and spasms
Fibro, in an of itself, can cause what you are describing and so can lupus
Also, a brewing infection will make me feel like that too. Due to my immunosuppression, infections/viruses make me feel like I've been hit by a truck for a few days before the obvious symptoms of the infection/virus hit
I do have a couple suggestions that will sound like the stupidest thing you've ever heard in your entire life. I know because when it was suggested to me, I was convinced no one who was suggesting it to me could possibly understand the pain/fatigue i was going through or else they wouldn't have made such a stupid suggestion. BUT----I tried it. And it wasn't such a stupid suggestion after all. It did help:
Try a yoga class. Specifically, a yoga class targeted for seniors. Can often be called "gentle yoga" or "chair yoga". Just tell your instructor ahead of time so you can ask for modifications.
I had never done yoga in my life. I don't have enough energy most days to do the simplest things like take a shower, or walk a block. But I drug my butt to a yoga class, and it wasn't exactly pleasant at the time, and I felt like a fool because I was the youngest person in a class full of grannies who seemed to have the flexibility of dallas cowboy cheerleaders compared to me. But it helped. It helped so much I did it again!
And...even better...pool exercises/aerobics. Low impact but very helpful, especially if you can follow it up with a whirlpool or sauna. I felt like a different person. At a local YMCA, they even have a specific pool "fibromyalgia" and arthritis water aerobics. You have to do it slow and ease into it over a span of weeks, otherwise it will cause tremendous initial pain as your body won't be used to it.
Another thing that is helpful are these things called trigger point balls and foam rollers. They are used often in physical therapy. You can substitute "lacrosse balls" or tennis balls for the specific trigger point balls in a pinch. There are various exercise methods built around the use of these balls as a myofascial release mechanism. One is the MELT Method, there are also videos on these on youtube to show you how to use. Don't get me wrong, they HURT LIKE THE DICKENS when you are using them, like yelp out loud hurt. But afterwards you can often tell an improvement, at least I have.
Another thing that might be helpful is a good massage therapist. Just any-ole massage therapist will not cut it, you want one trained in myofascial release and fibro techniques. I tried massage and it was not helpful and was about to give up, when by happenstance, I got an AMAZING person at the spa I went to. He did a massage with partial myofasical release, part swedish, and incorporated eucalyptus oil and ended with biofreeze. It was like heaven--my muscles were singing Hosannah! Sadly, for me, he quit and I haven't been able to find anyone else quite as competent yet.