I have also experienced shortness of breath, where I feel completely winded just from going up the stairs. I am a little overweight but not by much at all and have actually been losing weight, yet this shortness of breath persists. I have a couple of theories on why this happens, at least for myself. Hopefully this may help others experiencing the same problems, I am not a doctor but am in school pursuing a medical degree.
For me I notice that I am more winded when I am fatigued (didn't sleep well or just a severe fibro flare day), I also have pretty much non stop muscle spasms in my whole back, hips and behind my neck. When our muscles spasm, it causes more inflammation which in turn causes more spasming and tightening of our muscles. This will also effect our breathing, we have to oxygenate more air as our bodies try to release the spasm and calm the muscles and nerves. Any kind of pain whether it be spasms or nerve pain makes our bodies work harder, thus our need for oxygen increases.
I also have minor asthma, but this is an important factor to take into consideration because my asthma is usually triggered by stress and anxiety. When our body is in pain and there is a lot of inflammation happening, anxiety and distress is also heightened, when we are in distress our hearts will beat faster and our oxygenation increases along with it. When our bodies are able to calm down (say if you are laying down or sitting down as opposed to being active), our heart rate will normalize and the anxiety will also subside more.
Though there are many contributors to increased heart rate or heavy breathing (side affects of medications, environment, fibro itself, genetics, diet, caffeine, routine, etc.) these are the things that I have noticed for myself personally as far as triggers. Try to learn what triggers your attacks, keep a log and even show this to your rheumatologist or doctor and seek they're professional advice.
Again, I am just throwing some things out there and what are known triggers for myself, everyone's bodies are different in genetics, how we metabolize medications and even in pain tolerance levels.
Hope this helps someone and gives you some ideas to discuss with your doctor and how to regulate.
Blessings and prayers!