Hen with a large 'pimple' at the base of her tail feathers

We recently had a much beloved garden hen die after an undefined illness.  We noticed several weeks before she became ill that she had a large 'pimple' at the base of her tail feathers, which soon disappeared and we didn't think much more about it.  Today, I've just noticed our other garden hen has a similar pimple in the same place.  I'm worried this was the first symptom of our first hen's illness and am very concerned about it in our second hen.  Does anybody know what this might be, and what we should do about it?

OOOOO Thats what that is

Cos the ones on my 3 week old muscovys have suddenly swollen...probably cos they have started working