Missing fluff on butt

[D046113]

I have a small flock, only 3 adult hens and now two 5 week old chicks (they are still in the house, have not been introduced to the flock yet). 2 of my hens are on one side of the yard and 1 or the other so she is not picked on all the time. The 2 on the one side are both Red Stars and are about 2 years old. They have been very healthy since day one, and today they are both eating normal drinking normal, still full of energy as normal. One of the Red Star's did loose her fluffy but about 2 or 3 weeks ago, at first that is all I noticed but as of about 3 or 4 days ago now she stopped laying eggs... I don't see any gleet signs or mites... Any thoughts?

In the picture it almost looks like the feather quills are there but the fluff has been removed...