If it happens rapidly the hard to breath feeling is because ur feeling the process of ur body entering sleep paralysis quickly (not being used to it)that causes the muscles between the ribs that expand when u breath in to lock up and forces u to breath deeper. That can cause shortness of breath if breathing more from ur chest leading into the relaxed state.
To get out of body I Would stop worrying about trying to do it for a brief period and focus on other focus empowering exercises,like void meditation, fire meditation, meditation on ur senses, or just do foundation breathing meditation and practice lowering ur brainwaves at the same time by feeling urself like sink\fall\lower into ur self on exhales of the breathing rounds(sit up so u dnt fall asleep lol).
Also if I have any blocks in the major chakras try to clear them and the pressure feeling should go away.
My personal experience a blocked throat chakra and having smoking addictings can make the separation from body\spirit feel almost like ur stuck in an airless space and the only way to keep breathing is to pull my astral body(floating above me by like 4-5feet normally) back into my physical body to get air again. This is a result of my personal energy addictions and chakra blocks but who knows mby some one else has had similar harsh experience and wondered y.
