Weird bulge on 4 chickens. Chicken injury?

2 chickens are older, but not fully grown, and two are chicks, We let them all out with our almost full-grown ducks and noticed after checking on the little ones that they had been beaten up a little and had bulges of fluid on their right breast. then we checked on the older chickens which have been in with the ducks for a couple of months now and they had the same thing on the same side breast. I dont know if I should worry about this, if it is an injury, a disease or what. We got the eggs to incubate from my mother and all of her hens are healthy. please help