Is it at the end of her back, where her tail starts - and does it look a bit like a hamsters tail?? If so it's her preen gland, you'll occasionally see her niggling this gland to preen the oil through her feathers.
there is a weird grout on the back of one of my chickens, it long and stick up about maybe 1cm, it on her lower back in-between her feathers. it is like a pinkish colour!?!
