suddenly having such intense pain in both my legs, followed by sudden weakness in the legs is something new for me.
Always have any new symptom checked out by your doctor. It is easy for us to assume everything is the fibromyalgia, but there are many things that are not and that could be causing symptoms like yours, or any new symptoms. If you don't get it checked you run a huge risk of either missing a diagnosis of something that is in dire need of medical intervention, or missing out on finding out that it is something outside of FM, and something that can be treated successfully.
Honestly, exercise is over rated for anyone with this malady.
I am afraid I am going to have to let you know that this statement isn't accurate. (see below)
What's happening is barbaric in nature.
I suggested in your other post that you read my post about how to manage fibromyalgia. Part of that is learning, (and it takes time) to accept the facts of your life and work with your body to feel better. Thinking in terms of something being barbaric is not really helpful. Now, I am not going all Pollyanna on you, sometimes it doesn't hurt just to say it sucks. But if you fight fibromyalgia and think badly of it you are only fighting your own body rather than putting that same energy into taking the best possible care of it.
About exercise..................
Exercise is vitally important for everyone, and
more so for people with fibro.
What is important is the
kind of exercise you do, how you do it, what time of day, now hard you go at it, what you eat to support that movement, and other factors.
If you find out through gentle baby-step experimentation with many different things you will find what works for you in terms of exercise. You can start so small that you will think it's not even worth doing, (but it is) and build up in such tiny steps over time that your body gets accustomed to it. NOT getting enough exercise is the worst thing that a person with fibro can do, and it pretty much ensures that you will not be likely to feel better in the future.
Most people with fibro who do not exercise in some fashion find that they are able to do less and less as time goes by.
I have found that people who don't think exercise helped them did not go at it slowly enough and did not experiment enough to find out which of the thousands of different kinds of things you can do to get exercise worked the best for their body and mind.