Apparently I Don't Know How to Breathe

I thought I was a champion breather.  I breathe in, I breathe out,  I stop breathing if I'm underwater, I keep breathing when I sleep—it's very impressive, I know.

When I start meditating, however, more often than not, I quickly get out of breath.  Suddenly I can't take breathing for granted, it becomes a thing I have to try to do normally, and then I try to not try.  Before long I have to lean over and yawn, to catch my breath.  It feels great, but when I sit back up, it all begins again.  Meditation's got me yawning so much I half-seriously worry about dislocating my jaw. 

I’ve heard breathing “too much” can trigger hyperventilation, but the shortness of breath happens to me even when I slow my breathing down, even when I breathe from my  diaphragm.  It might be that meditation exposes some underlying anxiety, but this  is happening even when I feel calm and well focused.

I haven't cracked this just yet, but the good news is I don't think it's caused too much trouble.  The point of mindfulness meditation, at least at the start, is to focus attention on your breathing, however it occurs.  There's no right way to do it—you don't need to actively control yourself, don't need to take deep yogic breaths.  As far as I know, you can just breathe normally.  And my normal, for now, seems to be hyperventilation.

I'm working on it.

On a related note, when I do breathe normally, I find I have trouble focusing on it.  I generally feel only mild sensations in my nose and chest, when I breathe, and I’m pretty slim so my stomach barely travels in and out, when I’m sat upright.  Breathing, then, doesn't offer my attention much on which to focus.  I've found it helps to tighten my abdominals, because then when I breathe my diaphragm presses against the muscle, creating a sensation and, consequently, an easy point of focus.  I have no idea if this is a good solution.

If anyone has trouble breathing while meditating, or any tips to share, please comment below, I'd love to hear about it.  And I'll post about what I discover as I go along.