Airsacculitus?

My mother brought home some hybrid chicks a few weeks ago, they all seem to be in good health but this one has this strange air sac under its wings. This is present on both sides/symmetrical, not just the side shown. I'm seeing airsacculitus when I search this online, but I'm not seeing anything about the sac inflating in time with breathing. Is this indeed airsacculitus, or something else?

Thanks!