Yes, it will affect fibromyalagia pain, here are some reasons why:
an ill-fitting bra may also cause chest pains that mimic heart attack symptoms even when there are no cardiac issues at all. Too-tight, underwired or sports bras can add stress on bones and muscles by pressing on the rib cage, which in turn puts pressure on the diaphragm. Muscles that run from the upper part of the cervical spine and attach to the upper ribs can also be restricted by wearing tight bras. Sports bras should not be worn all day long for this reason.
It can also cause back pain and rib pain due to the constriction, and I would imagine in fibro, the pain would even be worse.
I was diagnosed now 10 years with Fibro and along with chronic pain there is so much more to it. I also have the sensations of what feels like worms moving around under my skin on my neck and back. This is unbelievably aggravating and because of this I have to take my bra off when I am at home. Wearing a bra is just too uncomfortable now for me and I am not a small chested woman. But you do what you have to do.