There are a couple of different kinds of muscle spasms in fibro, in my experience.
There is the intense, knifing pain of a huge spasm in a big muscle that lasts for a few seconds and leaves an aching behind, and then there is the consistent spasm in a tiny muscle that just goes on and on and is more annoying than anything.
Why is this happening? Well, based on my experience with curing fibromyalgia within my own body, I have a theory.
It took me almost 6 years to cure fibromyalgia because I didn't know that certain emotional states I lived in were continuing to cause fibro as quickly as I healed it.
But anyway, in about year 5, AFTER I started working with my mental processes too and hence started seeing huge and fast results, I discovered a new phenomenon in my body.
The new phenomenon was gentle, periodic muscle spasms in individual muscles - even the big ones - that were neither painful nor annoying, but kind of pleasant.
I did some research and discovered that the spiritual community calls these spasms kriyas, and sees them as the body kind of 'giving the muscle a good shake', the same way you would shake a rug from one end to get the dirt out.
This shake, in my opinion, facilitates the clearing out of stored stress chemicals, and hence signals healing.
So I love them.
However, they are WAY different from the spasms I remember back in my worst days of fibro - so what gives? I think that the body is always trying to heal itself, and when all muscles in a body are crammed full of stress chemicals like happens in fibromyalgia, many muscles are trying to do this shake all the time, but they can't quite manage it because of the clogged state the muscles are in.
So, this leads nerves and muscles and fascia to be constantly in a state of 'almost-spasm' or 'just-about-to-spasm' which, when it happens, ends up being the horribly painful spasm or the never-ending spasm.
What's the solution? Start clearing out those stress chemicals and relaxing the nervous system with simple stretches, recovery exercises, and calming mental processes.