Bump on Bantam's back

[Featherss]

Went to feed my OEG and saw a fairly large bump next to his right wing. I have no idea what caused it.

https://imgur.com/a/hjF8STW

He just recovered from being covered in mites/lice and also just got pullorum tested. He moves the leg and wing fine and acts fine otherwise. He is supposed to go to a show in a month.

Please help me figure out what this is and how I can fix it!

 

[Tookie]

How is your chicken? What does the bump look like now?