Small lump on ex-bats bum!

We got our 3 ex-bats on friday (thank you BHT) and 2 have bald chests but one has a bald bum!
she'd got new feather coming through as the lady had kept them for someone else who never collected them.
They have pecked at each other a bit so i got some anti-peck spray and some antiseptic powder.
When i was putting the powder on i noticed a small pea size lump just below were her tail feathers are on her lower back.
Could it be a blocked feather folical ? it looks a bit yellow.
 Any idea what it is or what i can do please?
 i don't want to pull her about to much but i don't want her in pain or anything.
Thank you

i have 5 ex batts and 1 of mine on her preen gland is quite high and lumpy too.  It doesnt worry her and hasnt got infected so maybe its ok.  This ones feathers are whiter than the other 4 a lovely golden brown colour?