I get this when I'm stressed. It started way back in college and completely freaked me out at first. I finally figured out that I was hyperventilating. Not the dramatic kind where you have to grab a paper bag, but more subtle - just can't take that deep breath. The more I'd try, the more I couldn't. Then I got obsessed with taking a deep breath and on it went...... until finally, success. Then it would start all over.....

Obviously, this may not be your problem, but try to distract yourself and see if it gets better. I also find it does NOT get worse with exertion. Probably because when you're exerting yourself, you're kind of distracted doing whatever it is you're doing and not focussing on breathing.

It hardly ever bothers me now, because I recognize it and try to unstress but you know how that can go. Sometimes I'm not even aware that I'm stressed until I start to hyperventilate.

I hope you get to the bottom of it soon. Whatever the cause, it can be frustrating and scary while it's happenning.

C