My chicken has had this big red rashy lump on her bum for about 2 maybe 2 and a half years now. Last year it looked like this and everybody said that that an egg was stuck inside her. She had that lump all last year, but when the weather started warming up, oneday she laid an egg. So we we thought she passed it. Then her feathers grew back over her buff and she was fine. But now that the cold months are here, her buff is all red,bare, and rashy with a weird lump on it..... Other than this, she is a happy, normal chicken although she is getting pretty old. She lays less eggs every week now but there are no other bad symptoms.
Has it grown in size? I have a few hens that get a naked butt like that, and I guess that it is dirty feathers that have either dropped out of or were easily picked out during preening. Sortof like the acid in the poo burns them off. I never had any feather pickers. The bumpy skin is probably normal unless you have a growth that enlarges in size. You can probably use desitin or diaper rash cream there if you want. It will protect the skin and sticks a long time.