I just started a topic myself about something similar on my leghorn. I thought the same thing you did. Looked like her pin feathers got trapped under her skin. Now the lump has died and turned necrotic. She has an open wound the size of a quarter. I'm at a loss.
Lou Lou, a year old Leghorn and the favorite of my 4 chickens, molted then as she fluffed back out, developed a large lump on the back of her head just below her comb. It was bloody from the other girls pecking at it so I cleaned her up with peroxide/water mix and dosed her heavily with...