I've never heard of CPAP causing water retention. Not saying it's not possible, just I've not heard of it.
There's a lot of reasons for it, one of which is the hot weather. So it could be pure coincidence, especially if you live in one of the areas being slow cooked. Some of the others are salty foods (but it takes a lot for most people), high blood pressure, prolonged sitting (like plane ride or long car trip), and kidney disease. And pregnancy, of course.
I strongly suggest that you see your GP about it.
As for aerophagia, not, it is not directly related to mouth breathing. It is air swallowing.
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Breathe deeply and count to zen.
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