Hey m2b,
my
guess would be that You feel the strain on Your epiglottis and/or neighboring muscles. A full inhale takes some effort to be kept in the lungs and it's plain muscle work to do that. So what You feel might just be fatigue or overexertion of the muscles involved.
These muscles can be trained. For example some pranayama excercises will do that, but there are non-Yoga things to do as well. If You try this and, after a couple of weeks, the feeling lessens or appears later in the hold, that would support this theory.
But again, please be aware that this is just a guess.
Please post Your results, maybe there's a lesson to be learned here.
