Maybe too much stress (you KNOW what that is!*bg*), too much to think about. I wouldn't worry about it, either, unless it were to happen constantly. I've done the same type of thing.

It may sound silly, but most people are shallow breathers and do not get enough oxygen, really. Try doing some good deep breaths each morning, noon, and night; remember a deep breath makes your belly go out like a balloon being blown up and go in when you exhale. If it happens again, try deep breathing in order to oxygenate your brain more.