Hi and welcome to the fibro forum, Alex! Did you read Fibro 101, the very first post on the forum? Lots of good info.
I always try to alternate "active" movement with something sedentary. That works pretty well. For example, I just did some laundry and folded some clothes and put them away. Now I'm sitting here, getting a little break. But I never can sit for long--as almost medfree said, we need to keep fairly active. I can stiffen up again quickly!
It can be easier said than done. I'm in a strong flare now, but pushing hard against it. I plan to keep doing active work for a while through the day, followed by a little couch time, but not a long time on the couch. When I'm there, I go through mail, or sort through magazines I really need to go through (yes, I did unsubscribe from a couple).
So really, it's up to you as far as whether or not you can do something. If it's something you enjoy, it can be a good way to distract you from the pain. Music, reading, crafts, and some prayer time help me every time.
I don't have much trouble with my knees, though occasionally I do get weird sensations there if I start to kneel. Could it be arthritis? Maybe you could get it checked out by a board-certified rheumatologist. I'm blaming this flare on our cold, damp weather. I haven't been "officially" checked, but I do think I have a bit of arthritis, especially in my hands. I go into a flare without fail every late winter, and it's better once the snow melts and things dry up a bit. I just keep trying to push through it, though gently.
Hope you have a good day, and again, welcome.