Newbie with puffy cheek problem

am SO glad I found this website! Great articles, well-indexed posts, upbeat we-can-do-it attitude. I am 3 weeks into learning to live with CPAP.
Problem: I get pockets of air in my mouth, like chipmunk cheeks. When it's happening, it happens every few minutes and is most annoying and wakes me up. Sometimes I go three or four hours without the symptom.
I read through the posts about this from last April, and tried putting my tongue to the roof of my mouth. (Thanks, LSAT!) It worked great… at first. Then about 3 AM the chubby cheeks and air puffs were back and no amount of tongue on the roof of my mouth worked. I tried: Taking the mask off, giving it a half-hour rest. Using my asthma inhaler. Deep breathing. Wearing the soft cervical collar. Nothing worked. Any ideas?
I can't lie flat on the bed (someone's suggestion) because of acid reflux. I'm using a nasal pillow mask. I tried using a full face mask, which almost eliminated this problem, but I can't use it because the anchoring cross on the full face mask rests on the back of my head right in the place where I have had two surgeries for Chiari Malformation, and the mask straps cause major neck pain and headache. I went shopping for different full face mask online, but I couldn't find any that didn't have that same cross anchor. All suggestions welcomed! It is so nice to find that I am not alone with this journey.
Nancy in Colorado
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machine: ResMed Auto Set S9
setting: 6-17 (I think)
Slimline tubing
humidifier (but not the heated tubing set up)
humidity setting: 3.0
mask: ResMed AirFit P10 nasal pillows
have tried both small and medium pillows; same result