I also have Fibromyalgia and like you have days when I feel very weak. I combat this by doing a mixture of yoga and bodyweight/high intensity work.
I do the latter in short bursts of maybe 6-10 mins at once/ twice a day.
I try to aim for 3 yoga sessions a week. 2 of those a shorter flow style to build strength and 1 long yin session to release tension in my muscles.
Plenty of water and fresh air to keep me awake.
It doesn't magically take all the pain away but it helps and it keeps the fog away. I always feel worse when my brain is foggy.
I don't know if this helps at all but it's how I try to manage it.