I have this problem too, as does my best friend. Occasionally we don't take a breath in 20-30 seconds, but as soon as we realise, normal automatic movement kicks back in (after a deep breath or two).
In me, it seems to be a problem with focus, only it doesn't happen when I'm focussing too hard on something else, but rather when I'm not focussing on anything at all; I zone out of everything, apparently including basic bodily functions. Her case is less simple: sometimes total absence, & sometimes too much attention on something else.
We're both 20 years old, & I'm pretty sure we're both deficient in quite a few essential trace elements, zinc & magnesium in particular. We're also pretty anxious people, though it manifests completely differently; the problem with that explanation is that this happens even when we're completely at ease; indeed, more often in such circumstances.