pus inside lump

I was just holding my 4 month old Lavender Orpington hen when I noticed that the feathers didn't look quite right on her back, just behind the neck. I ruffled through her feathers and found a bump filled with pus... I was able to clean her up and disinfect the wound, but I'm very curious as to...

I noticed a lump growing above the tail and now the lump has turned yellow on its sides (pus inside) as you can see in the pictures I uploaded. The chick eats & drinks alright but feels more sleepy than the others & not as active. And also keeps his/her feather fluffy all the time, like if...