Lots of anxiety sufferers go through this and I've had this symptom for decades. Its a type nightmare, a sleep abnormality really, caused by anxiety. Sort of like the nightmare of feeling paralysis during sleep.
Basically what happens is that your not suppose to be aware of your breathing slowing down while falling asleep but for some reason, caused by anxiety, you are. This immediately jolts you awake by giving you a smothering feeling like you've stopped breathing but you haven't, your just aware that your breathing has slowed.
I have spent years analyzing this symptom on my myself and testing what its been, when its happened to me and what i described above is exactly what is occurring. It usually occurs when your sleeping in an awkward, unusual or uncomfortable position or if your having indigestion, a headache or any other physical disturbance.
Its normal for us with anxiety and completely benign. It will go away on its own.