It is rather unlikely that you had slowed your breathing to the point where your body started to spasm. It is an automatic nervous function, and generally unless there is some physiological issue going on the body will simply not allow itself to reach this point.
That being said, many people do experience pretty intense sensation along these lines. I theorize that when they are seeking to astral project in this fashion they unintentionally seek a lower vibration closer to their life essence... Or in other words, they accidentally work to state of etheral projection. This state of spiritual awareness, as I understand it, is much closer to the nature of the life essence, and as such tends to leave us with more physical symptoms of the projection, and in some cases leaves people more exhausted or with more difficulty than astral projection.
I might recommend seeking a higher vibration... something that feels "lighter." One method I recommend for this is to observe a flow of energy that you enjoy, relax into a trance within your study of it, and allow yourself to simply "drift" in it.