She may be an internal layer or have a hernia. How does the mass feel--soft and spongy, firm, or tight like a drum? If it is tight, she may have fluid or ascites in her lower belly. I have a hen who looks like that, and she has to be cleaned off of droppings occasionally since it catches them. My hen has lived for a couple of years, but she does not lay eggs, and her belly is probably full of lash eggs, egg matter, or fluid. Occasionally, some hens have hernias there that can rupture and spill out intestines and abdominal organs. These types of enlarged masses in that location are fairly common.