Greetings
Rivington,
It's hard to tell from looking at a photo, but, I am guessing it is an injury.
I think I see some small bits of dried blood. Perhaps she cut her mouth on chicken wire, as, there are lots of unseen protruding pieces of wire in the pen. Or, she simply got pecked by another chicken. It doesn't look like a tumor, or cyst, however the lower part can be abscessing from infection.
If an abcess forms, you will have to drain it, and treat it as an open wound. Flush with peroxide (one time only), or flush with saline solution, then, apply antibiotic ointment.
I would start by treating it as an infection. Clean the area with some peroxide (one time), if the wound has been
untreated for a couple of days. Otherwise, if it is a fresh wound, flush with a simple saline solution. Finish, by applying an antibiotic ointment, like: Neosporin, Vetericyn, or Colloidal Silver. Apply ointment twice a day. Clean when needed.
If you don't see improvement within 48 hours, another diagnosis will have to be considered.
I hope I have been helpful.
Other members will have advice, please consider these too.
God Bless
