It's very hard to see in the photo, but based on the location, I believe it is the preen gland, the correct name is the uropygial gland.
Most birds have this gland. A waxy oil substance is excreted from it, and the bird uses its beak to spread the substance onto the feathers. You may have noticed the chickens poking around the base of their tail and then, moving their beak through their feathers as they preen and fluff.
I does look like a pimple or mosquito bite when they are little. As they grow the gland will too. And it will look sort of like a tiny baby bottle nipple! LOL!
If the gland is bleeding the chick may have scratched it, or another chick may have been curious and pecked it.
Keep an eye on it, if it scabs over, it will be fine.