I just started this fun new little symptom a month ago. AFTER I stopped smoking! Hows that for a kick in the pants! Anyway, I did a little research and found that anxiety sufferers are typically overbreathers. We aren't really short of breath but by overbreathing and shallow breathing (chest breathing) we have thrown our O2 and Co2 levels off.
I posted about this not too long ago and have a link for some exercised and info that really helped me. It took me a couple of weeks to acclimate to the new symptom and get it into my anxiety raddled brain that I wasn't about to suffocate (not a pleasant thought or feeling) or die. Once I did, I stopped reacting to it so much. Sure it bugs me but doesn't scare me to death anymore.
Here is my other post: That is so bizarre that you posted about this because I have been going through the exact same thing for the past couple of weeks.
From what I can find it's called "air hunger" kind of feeling like you need to take a deep breath. Not quite "shortness of breath" but close. Yeah, it's had me scared too. However, I have also just quit smoking so my body is adjusting to breathing in completely different patterns as well as having a different O2/CO2 combination. From what I learned it is too much CO2, that we are actually overbreathing. I know it's not anything scary like my heart or a lung problem, cuz like you, if I'm distracted I'm okay. Also, I exercised a few times while I seemed to be having it to see if was cardiac related. And, exercise actually seemed to help. In other words, if it was something bad it should have gotten markedly worse as I stressed both my heart and lungs. This little trick seemed to help: take a normal breath in, hold for a count of five, breathe out slowly and then hold for a count of 5. They said it was overbreathing which is VERY common for anxiety sufferers.
Here is the link:
http://www.rbphd.com/Docs/breathingretraining.htm
Mine also comes and goes and I've gone 3 days now w/o much problem. And, its always worse when I feel anxious.
I have arrived at the conclusion that its' just another anxiety symptom that can be put in the "oh, that garbage again" pile.