Since she's acting fine otherwise, I would leave it alone for now and let it heal.
If it gets bigger, then you should probably pop it.
Here's a thread about ruptured air sacs.->
https://www.backyardchickens.com/threads/ruptured-air-sac-subcutaneous-emphysema.1432594/
Oh yes, definitely hard to get pictures with all those feathers, I understand! I was just thinking pictures might help.
I was thinking it could be an air pocket, which is from a ruptured air sac.