My best guess is it was attacked by an animal and it healed abnormally. How did it start? ...was is a small growth that kept growing in size or you just noticed that huge bulge one day? I would clean the area with warm water and soap and disinfect.
Keep her separate in a clean pen with clean shavings and put her on a little antibiotic maybe. That looks pretty bad.
Keep her separate in a clean pen with clean shavings and put her on a little antibiotic maybe. That looks pretty bad.