If it feels like air then it's likely a ruptured air sac. Sometimes they will resolve on their own. If it continues to get bigger, or the chick has trouble breathing, then you will need to release the air. If you sterilize the needle, disinfect the skin where you prick it, and apply a tiny dab of plain neosporin or plain triple antibiotic ointment to the spot you prick, then risk of infection is low. Not zero, but low. Sometimes you may have to do it more than once, as they can refill until the air sac has time to heal up. Releasing the air trapped under the skin takes the pressure off the air sac, allowing it to heal up. Chicks can rupture air sacs running into things, each other, falling or being dropped, or just getting knocked around.