Hi there -
What you describe sounds a lot like pleuritic pain, which is usually due to an inflammation or irritation of the chest wall, where the pleura (the lining around the outside of the lungs) moves against the chest wall. If the pleura or the chest wall surface muscle become irritated, a deep breath can feel like being stabbed. The pain will often radiate all the way up into the neck on the affected side. If you have a chronic sinus problem this could be part of the cause for sure, and having fibromyalgia would only make you more prone to this sort of thing.
The good news (I hope, anyway) is that this usually responds well to non-steroidal anti-inflammatory pain meds such as ibuprofin (Advil) or naproxyn (Aleve). Since you are currently taking Prevacid for GERD, you might want to be careful with either of these drugs since they can irritate the stomach. It's kind of a catch-22, although some people tolerate the NSAIDs in spite of GERD problems, maybe because they are taking something already for that.
There's not much else in that area, just the lung and its lining, and the chest wall itself, but that's sometimes enough to cause excruciating pain, if either becomes irritated. Given your existing medical problems, this seems extremely likely.
I hope this is helpful to you. Best of luck with this, and please follow up with us here as needed.
John Kenyon, EMT, CCT
Non-invasive cardiology tech, Emergency and Critical Care technician, Critical Incident Stress Mgmt. specialist