Hen not 'normal'

Hi again,
One of my laying hens, a cuckoo Maran, isn't 'right'.  She hasn't layed for 4 days, and she never misses more than one day.  She is so amazing she layed for 16 weeks straight!  I picked her up the other day, and she was feeling a bit skinny.  Then i picked her up again and she is really underweight, her breastbone is really sticking out, and she has lost a few feathers. We got her at POL in May last year, and she never moulted.  She has simply lost feathers on her botton and her chest area.  I haven't wormed her with anything but Verm-x, but I read that this wasn't the best.  We have ordered some Flubenvet-is this better?
She is eating and drinking fine, and still perching fine at night.  Does anyone know whats wrong with her?
Thank you 

Okay, thank you!  I checked for lice and redmite, but none there.  I will worm her then and if she is not better after that then I will take her to the vets.  Luckily we have one not too far away.  Thank you!