The area below her vent looks like a hen that I have who has a large sack of tissue that bulges out under the ven in the lower abdomen. Others have posted about similar "large red ball below the vent" which may be fat, a hernia, or other unknown bulge. It looks to me like she has been pecked or injured, and the wound has gotten infected and maggots have infested it. That is known as fly strike. I am not experienced with it, but from many threads on BYC, there are tons of information to read. The best treatment is to bring her into the house or garage where flies cannot get to her. Soak her in either Epsom salts, salt water, betadine, or soap and water in a warm bath to her bottom. Look for maggot larvae, gently scrub the wound to debride or clean off the wound scab. Get out any pus, and tweezers may be needed to get most of the maggots out. Repeat the bath again a couple of times a day until all maggots are gone, then dry it and apply plain antibiotic ointment (bacitracin, neosporin, etc.) Keep us posted on this thread how it is looking. Make sure that she is eating well with plenty of protein, and drinking fluids.