This really sounds like an air sac inflating. Airsac inflation can be caused by an injury to the airsac, or an infection causing inflation of the airsac.
Sometimes the infection is respiratory and spreads to the air sac, or sometimes it's environmental.
First thing I would do would be make good and sure that all their bedding, etc, is very clean, free of droppings, dry and fluffy. It sounds like the thoracic air sac. From front to back, is it somewhere between where the wing connects to the body and the middle of the spot under the wing?
Without being able to tell what exactly is causing the air sac inflation, you can actually deflate it daily while you try to determine a cause. Clean the area with iodine and water or alcohol. Use a hypodermic needle, prick the air-blister and pull air out. Repeat for a couple of days and let us know if it gets LESS inflated daily.
In the mean time, we need to figure out what happened. Any chance of an injury there? Any signs at all of respiratory illness?
When you say she's puffy, do you mean all her feathers are ruffled? Or just that area?
I would wonder if she doesn't have a systemic infection that is manifesting itself through the inflation of this air sac.
But your answers to the questions about will greatly help us go further from here, so I look forward to your reply. Thanks!
