This might be from an injury. If her skin was torn deeply enough to tear the fascia near her lungs this might be what is called "Wind puff" This is a common complication in birds that have been caponized. The cure is easy - just pop it like a balloon. And it will feel VERY MUCH like a balloon.
It is way too large and in the wrong place to be the crop. And it's also in the wrong place to probably be an issue with being egg bound or an infection in the abdomen. AND she's way too healthy looking for it to be a massive infection .... although I could be wrong on that, and if that is what it is it would still need to be drained.
SO. Gather up some towels and your courage. Get something like a thin pen knife, and some alcohol or vetrycin (preferably the vetrycin) or perhaps betadine if you have that on hand. Hold the chicken gently but firmly, and pop that balloon!
If it's wind puff, you'll probably have to pop it a few more times before it heals, and be sure to monitor for infection (unlikely, but possible) BUT she will feel SO MUCH BETTER the second you pop it.
And if it is some kind of weird infection, then .... well.. you won't have harmed her by trying to pop a wind puff, and you'll know by any liquid drainage that it is an infection. AND and and ... if you don't get wind or puss or anything else draining, then you'll know its a tumor of some sort and you'll have to make some other choices.
tl:dr - Poke that thing and see if it pops or drains.
Editing to clarify: Only go in just enough to go under her skin. A half inch, tops. If it is wind puff, as I suspect, don't be shocked by how much air whooshes out. AND, again, if it is wind puff, leave the wound open. Don't attempt to stitch or bandage it. It's going to need to keep letting the air out for a few days while the lung injury heals.
