Help with Cheeks Puffing Out

My 79 year old Mom just started on cpap a few weeks ago. As an old pro of three months, have been able to solve both our problems so far thanks to this forum.
She has a Resmed S9 autoset, and is using the Nano mask. The doctor prescribed a pressure of 5-14, however we have adjusted. She is a severe asthmatic, and at a pressure of 5 she felt like she was choking. We uped the minimum pressure to 7, with no ramp, and solved the choking problem. We set the exhalation to 3, since she felt like she had to work too hard to breathe. We solved the runny nose problem by switching to the climateline hose and a temperature setting of 80 degrees. The messed up hair issue was solved by switching to the bella loops.

Initially, she would put on the mask and fall right asleep. For the last several nights, she has been having a hard time falling asleep since her mouth is filling up with air (puffing out her cheeks) and she feels like she has to open her mouth to let out some of the air. She used a chin strap early on, but is not using it now.

Any suggestions? Thanks!

I'm 72 and had a similar issue. A chin strap does help. Can you convince her to resume using a chin strap or perhaps try a different chin strap. Unless she is suffering from dementia she should be able to adapt in time. When my cheeks puff out, I just squeeze the air back out through the lips and close my mouth tight with my tongue up against my teeth. However without a chinstrap it can happen as soon as you relax into sleep waking you up. Another solution might be a mouth guard but that would involve more adjustment.

chipmunk cheeks is what we call it.
Do a forum search for it and you will see some ideas on how to deal with it.
Mainly the issue is the tongue not blocking the door to the airway and air from the airway enters the mouth...lips are closed...cheeks inflate.
The trick is to keep the tongue in place to keep the door closed. Easy to do while awake but when asleep the tongue gets lazy and drops down and opens the door. So we have to train the tongue to stay in place and keep the door shut. Usually tongue to roof of mouth will get this done. Practice during the day every chance we think about it and eventually it becomes habit.

A full face mask will usually fix this...if she can adapt to it.

Regarding cheeks puffing out. I have found some success in taking one of those stirring straws that they have where they serve coffee. It is like a drinking straw but with a much smaller hole. I cut it in half to about 3-4 inches long. I keep a couple by my CPAP (so if I lose one in the dark, I can reach for another). When I begin the "puffing" I put one of the straws in a corner of the mouth and let one end go past the teeth to about the center of the mouth. When the air starts to build in my mouth, the straw acts like a pressure safety valve and lets it out gradually instead of it building up. I have fallen back asleep. Sometimes I will wake up with it still there ... or it will be nearby on the pillow. But I got the needed sleep. This usually works fine. At first I worried that it might go down my throat and gag me, but the lips at the corner of the mouth wrap around it fine and keep enough pressure to keep it in place and not go any further in than necessary. You might try that.

Another possible position for the tongue is to push just the root back a little and relax it there;
the back of the tongue touches the back of the roof of the mouth and the rest of the tongue relaxes or moves (e.g., licks lips) as it pleases.

Another way of saying this is to just relax the tongue backwards almost as if you are (GENTLY) swallowing it.

I don't know if this is hard or if everyone can do it -- it worked for me right away. YMMV