deformed Duck Advice please

I have noticed this morning that one of my 6 week old ducklings has a deformed lower neck/chest, on the ducks right handside his/her chest pertrudes by about 3/4 of inch verses the left handside, I dont think this is something recent I think I saw iot a few weeks ago but assumed it was just the angle I was looking at her, but wy wife just pointed it out me and it looks quite significant, its probaly at little larger that half an egg but does look smooth not like she has swallowed something.
She/he is eating well and acts completely normal!
Has anyone seen this before, what do you think it may be, will it go away or is it there for life, could she be in pain.

Any advice would be appreciated

is it right to be on one side and not the though?

Thanks Kate, you can tell I'm a novice