There is something called a breast blister. It is caused by the breast rubbing on the edge of a waterer or feeder etc.
http://www.welphatchery.com/poultry_health.asp
Oh and to answer your original question. It does not sound like fowl pox.
Imp
Is the lump on the right side between the neck and chest?
If so, it is the crop, and is perfectly normal. The crop is where they store food, before digestion. It will go down overnight, as they digest. It will fill up again as they eat.
It could be a problem if it is hard and doesn't go down...