I would agree that it looks like a breast blister which can become an abscess. They are common when chickens roost on the ground, especially if bedding is damp or rough, or if the roost they sit on is rough. Clean it with betadine or Chlorhexidene and apply som plain antibiotic ointment daily. Make sure the roost is covered or the bedding is dry and clean.
And her stomach feels to be a bit better hopefully. I will update you guys tomorrow. Good night! 
