I am not really agreeing with boulderjohn on this one. Sounds more like hypoxemia and yes your body does have a means of detecting low arterial po2...they are called Peripheral chemoreceptors. Rapid breathing does not cause a CO2 build up...not breathing or not breathing properly does (breath holding or skip breathing). why do you think that free divers like to hyperventilate...they blow off CO2 and wait for the low arterial po2 to stimulate them to breathe which is a very good cause for shallow water blackout. So by stopping all additional movement does not help you get rid of CO2 but keeps large muscles from burning more oxygen. It could be a mixture of both but I would stand that it was more of a lack of oxygen than a build up of CO2. OBTW, what FiO2 were you breathing?
I have to take issue with this (and, btw, I'm a physician with substantial critical care training).
We DO have a mechanism for sensing low arterial oxygen tension, but it is weak and poor, and nowhere near as powerful as the drive to keep CO2 down. Freedivers DO hyperventilate to get rid of CO2 before diving, but they don't do so they can "wait for the low arterial pO2 to stimulate them to breathe". They do it so they can avoid the CO2 limiting their time underwater, and the DANGER of it is that they will pass out, because they DON'T have warning from the low arterial pO2 to tell them it is going to happen.
Rapid breathing does not necessarily result in CO2 retention, but rapid SHALLOW breathing absolutely will. This is because the ventilation with shallow breaths is largely confined to exchanging the gas in the large air spaces, which do not participate in gas transfer to the bloodstream. On Scuba, if you breathe very rapidly, you will almost of necessity be breathing shallowly -- this is because the work of breathing is higher through a regulator than it is sitting in a room. The body tolerates increased work of breathing poorly. We are designed for breathing to be almost effortless, and when it isn't the person gets acutely uncomfortable. I would suspect anybody who has used a snorkel has had the experience of trying to chase some critter and getting very out of breath . . . this is because the snorkel increases dead space, and to maintain the same level of exertion as on land, one has to move much more air, and the work of breathing is very unpleasant.
Exerting yourself excessively on scuba will almost certainly result in CO2 retention. CO2 retention makes you feel anxious, and makes you breathe harder . . . and on a regulator, that translated very quickly breathing faster, but not as deep, which results in even MORE CO2 retention.
CO2 is the issue here. Remember that the air you are breathing at 4 ATA has as high an oxygen content as breathing through a high-flow mask of pure oxygen. It is difficult to get hypoxemic at depth, unless you are using a rebreather and not delivering oxygen into your breathing gas.
