Chick with tumor or bubble sack in front of wing?

[KayasChicks]

I was taking care of my chicks this morning when I noticed one of my youngest chicks, which also happens to be a bantam, has a weird growth or something on it. It's been eating and drinking fine and can even move its little wings pretty well for its age. The growth is like a bubble, soft to the touch and doesn't seem to bother the chick much when I was watching it just now. Any ideas on what it could be?

 

[YellowRoseFarm]

I've read about those, and it could be a cyst. A bacterial or fungal infection could cause it, but I'm sure one of the experts can weigh in.