your new hen, in the other thread-why was she given baytril and did she have the full course?
It could be gapeworm -I lost a few to it last year despite having wormed a month previous. If the hen is lower in the pecking order or if they were free ranging while having the flubenvet pellets she may not have had enough. Gapeworm is horrible so if you think it a possibility I would worm them all again-restrict them and maybe put the flubenvet pellets in a couple of different feeders so everyone gets their fair share. By the time you see them gaping (and it is actually gapeworm) its 50/50 whether they survive even after treatment (I lost 3 after treatment, 2 survived) so even if the others aren't showing signs yet, they have probably picked it up. If you have a lot of crows/starlings around, well they carry it.
It could be that the new hen bought something in with her-not necessarily what she was being treated for either and it might not be serious. However, its why many of us don't buy in stock or if we do, we quarantine for several weeks.