I've always done this.
My friends would often make fun of me for 'forgetting to breathe'
It sounds pretty stupid when I put it that way, though...
'why are you gasping for air suddenly?'
"...uhh... I forgot to breathe." -__-
dur.
It's a problem I've always had when my anxiety is high around crowds of people. I tense up and hold my breath-- as if holding my breath would make me invisible or something... which is obviously doesn't but doing these things has become a habit that seems impossible to break. It's reflex, almost.