There's a spot under my daughters chin that's red and kinda looks raw and it stinks. It's worrying…

There's a spot under my daughters chin that's red and kinda looks raw and it stinks. It's worrying me I have no idea what it could be from. It doesn't hurt her. I be sure to put something under her chin every time I feed her so milk doesn't get down there and if some does I wipe it up and I clean under there when I give her, her baths. What could it be?