Let me guess! You are the 'natural mouth breather' not a 'nose breather'. Some of the divers, mostly the ones that preferred breathing through the mouth on the surface, experience that issue diving on chest mounted RB's without OPV. It was explain to me by some navy physiologist, that this is just a variation of 'chipmunk cheeks' caused by difference in hydrostatic pressure between the counterlung and your oral and nasal passages. It has something to do with muscle strength - in 'mouth breathers', during the ascent, specially in a vertical position, instead of becoming a chipmunk, your weak nose 'sphincter'
muscle burp an excess of a gas. Resolution of this issue is to achieve better trim -try to stay horizontal most of the time. Other important thing to avoid ascending with a full loop. Slow down on ascent, let oxygen get metabolized, in non hostile environment you can vent some gas out through your mouthpiece(not an option in combat).Hope it helps.
rgrds
Tomek
