Zebra hen, plucking tail feathers and has tiny white bump (puss?)

by Marty
(Brockton, MA, USA)

Hello, I just noticed yesterday that my otherwise very healthy looking Zebra has a large bald spot down her back close to her tail. The tail feathers look fine. She has nipped at her back area, almost like it's irritated. Also, there is a tiny white bump that looks like a sore w/ puss in it. She is acting very normal, is eating and jumping and flying around the cage like usual. I have two other finches in the cage and they look fine. One of my hens is laying eggs but usually both hens lay at the same time. Could she be egg bound? Her behavior seems to indicate that she is not as she is still very lively and not lethargic
Thank you!