I agree that it sounds like anxiety. The feeling hits me out of the blue when I'm not even stressed. I could be totally relaxed watching TV. I feel like an elephant is sitting on my chest. Deep breathing exercises can help. Breathe in for a count of four, hold the breath for a count of four, and exhale for a count of four and repeat a few times. Also if I get up and change my surroundings or do a chore to distract myself it seems to go away. Remember - Having Fibromyalgia means you are stuck in fight or flight mode so physical feelings of anxiety go hand in hand.

You could also be having an anxiety attack. I've had them for years and they can be scary

Yes! I often take these huge sigh-type breaths and my family thinks I'm strange but it's the only thing that feels kind of better. Breathing through just my nose, I feel especially like I'm getting little air.

I'm adding another answer today LOL.. I was looking through different little blurbs that describe all the symptoms of fibromyalgia to send a loved one so that they understand what we are going through and 2 have shortness of breath on it. Like its a common symptom.
I believe that's what our pin ups are for?
I'm going to post it there. If anyone wants to look at it...

Hello, I have that feeling.like you're not getting enough air in you lungs. Well guess what that is Anxiety, no bull that is what it is. I've had it for years and just summed it up to my asthma until a doctor at the pain clinic I went to noticed it. Now I know when I start doing that I have to relax and destress and breathe. You can work through it, mention it to your doctor.