It feels as though the diaphragm may be affected because the ribs won't expand during flare ups... almost like everything in this area is frozen, can't move much at all, and deep breathing is extremely hard to do.
Shortness of breath was a problem for me early on. I was fairly active at the time with good endurance but it seemed liked my chest wouldn't expand. My rheum called it costochondritis which is inflamed cartilage, especially around the breastbone where the ribs meet in the front of the chest.
Shortness of breath continues to be a problem for me and I still to tell my doctor about it. Now my doctor says it is multifactorial and I have to agree with that assessment.