Sorry that I won't be much help. Out of our freshly deliver box, one had a lump under the tail and under the vent. Cleaned it off since it was covered in poop and I thought it was poopy butt and it was red and swollen.
4 days later, looks like an umbilical cord that dried out but hasn't fallen off. The chick is eating and growing, still small relatively speaking, but getting along with others. For that matter, all the Plymouths are smaller than the Buffs and Reds. She is the only one with the lump.