I'm not sure if this is what it may be, but I read that when a chick is born, it has the yolk sac to get nutrition from for it's first few days. That's how they can "mail" the chicks within the first days. During those few days, the yolk sac gets smaller and it's not as easy to notice after it has been absorbed. Maybe the yolk sac was slow to dissolve on this one chick? I noticed one of my baby BRs had the same bulge and now it's gone one week later. Just a though...
