I have a Turken that has the same thing... but you can see it all the time because she has no feathers there.  We thought all the time that it was a big tumor... gross.  We even named her fugly because of the way she looked.  But I've noticed that if you watch her, the "tumor" will be very small in the morning.  Then she will go eat for while and then it might be really big.  I think the other post was correct.   I think it has something to do with the crop... maybe sometimes its really full, and sometimes not.  Watch the size of it when she eats.
fugly is our favorite chicken now.  good luck.