Not headaches, but I would start to feel faint/dizzy during (causes a drop in blood pressure) and a day or two later, intense pain in any of the muscles I worked, and I would feel like I got hit by a truck. Like my limbs weighed a thousand pounds and I couldn't lift them. Even something as simple as tearing a piece of paper would feel like an immense effort. Early on, when I first got CFS, it was so bad there were several days when I couldn't hold my head up without crying.

This symptom is officially called "post-exertional malaise," which doesn't even come close to capturing the horror of the reality. The cause is that in people with CFS/fibromyalgia, the muscles don't replenish themselves with glycogen and ATP the way they ought to. When my FM was bad, I used to take D-Ribose as described in From Fatigued to Fantastic and that did help, but mostly I don't need it any more. I do much more physical activity now, and when I overdo it, I'm tired, but it's not disabling.

I try to avoid pain killers when I can. I haven't found that they change my sleep any more than the magnesium, which doesn't have the side effects.