Hi- I also have that "wonderful" Fibromyalgia/Sjogren's combo.
Along with some "unspecified arthritis" according to the rhuemy.
I think how and when to exercise has been one of the toughest questions for myself. As quietdynamics wrote, "movement boosts serotonin" ;that is true. However, on days when I can hardly get out of bed because of back and joint pain and stiffness, I'm not on my way out to take a long walk. If you can, that's great.
Personally, I've come to the point of doing what I can when I can. I can't schedule anything since I never know what I'll feel like from one day to the next. A few days ago, it was warm and sunny here in Kentucky and my husband and I took our dog for a walk. I was a little stiff, but got started and we went around 3 blocks. He and I were both amazed I could do so! And it felt good.
Today, it's rainy and I hurt. Making dinner, a load of laundry and cleaning up the dishes have whipped my butt.
I would advise not to push yourself too far. If you do too much one day when you feel good, you could feel awful for days after. Best advice is to start with something gentle; a short walk, a few stretches, etc. and see how you feel. It takes some time, but you will learn what your body can handle. Everyone is different.
I hope this helps a bit.