Maybe a breast blister? I dunno, sure hope someone more knowlegable can help you. I have stopped deflating my chick, but it still has a mass, but now it seems to be filled with something else. I'm unsure if it's fluid or crop, so I'm just going to let it be since it's eating, drinking and acting somewhat normal now.
The problems with my chick are all my fault, I had helped it out of the shell as it was late and it was positioned sideways. It never absorbed it's yolk, which I had to tie off because it burst in the incubator from dragging it around. I thought I had secured it enough in a damp paper towel, but when I returned home from church, it had punctured the yolk and also pulled out some of it's intestine. After I tied off the yolk and cut it off, I pushed the intestine back in with a wet qtip and smeared the navel with bag balm. Next day, the navel was healed it pooped so all was good there. Then that's when it started having problems with the air bubble. It was expanding everytime I had fed it with the syringe. I felt it was all or nothing, so got out the syringe with the needle and on an angle, put it in and then gentle pushed out the air through the hole.
Man, I felt so bad. Really felt like I was torturing the poor thing as I had to do it several times for 3 days straight. I hand fed it egg yolk, yogurt, vitamins and apple sauce mixed with a crushed antibiotic that I had left over from one of my cats that had a bladder infection. Now that it's eating on it's own, I'm gradually making it thicker with the medicated starter.
One thing that I think was a major cause of it's will to live was that after it could stand on it's own, I put in my other chick that hatched a couple days before it. It was exactly 2 times heavier than this chick but now they are both gaining at the same rate (I weighed them the last couple days). It follows the bigger chick around constantly, won't leave it's side. I think that if it had been left alone that it would never have made it.
I hope my story has helped you in some way, there is hope to heal very sick little ones, though the amount of time and energy involved is great and not always possible for some. Please keep us updated on it's progress. I should post a pic of my two together to show you the difference in size, it's amazing.