Are you certain that it is not her crop? Does it feel like air trapped under the skin? There is a condition called subcutaneous emphysema or a leaking air sac, that can be common after an injury to the chest or neck. It can be decompressed with a 22 gauge needle, and then air is pressed out. it may happen again, and that can be repeated. If the chick is not in any distress, I would wait and see if the air doesn't reabsorb on it's own. Of course, if the chick is having labored breathing or clicking, this procedure should be done. Let us know what happens. Here is a link or two to read:
http://www.ijasvm.com/ijasvmadmin/upload/IJASVM_5113cbc51d31b.pdf
https://www.backyardchickens.com/t/171121/deflating-subcutaneous-emphysema