I know exactly what you mean, although mine is a disk and nerve issue, not fibromyalgia I don't think. But anything that presses on my lower back really aggravates it and other places as well. Skinny jeans, skinny anything, just not going to happen in my world.
Also I have majorly damaged feet from other injuries, that the board has heard about ad nauseum, so I have very few shoe options and most of the time just wear Brooks. I know they ruin my look, but hey, walking matters too!
Anyway, a few days ago I posted some super great pants and the consensus was that they did not look horribly pajama-esque and I could get away with wearing them in public. Here is the thread, I did link the pants further down.
http://youlookfab.com/welookfa.....uch-volume
How I cope is to just do the best I can. I envy those who can wear what they think is pretty and not have to factor in what their health can tolerate. But that's not me, and I just try to look the best I can within the given limits.