Help please, black lump by comb

Alice, aprox 18mths old, good layer, has developed a black lump at the back of the base of her comb. I thought at first the comb had been torn somehow and it was dried blood that I could see. Now there is a black lump about the size of a black current, it also looks as if some of her feathers are missing in that area. Generally she seems quite healthy, eating, drinking, running around, laying.
The rest of our girls, four in total, are fine.
All advice most gratefully received, thank you
Penelope