Couple of things you have mentioned, in lay 3 months from POL. Also nothing obviously wrong. I am sure you have checked through the post by Chris for any parasitic evidence. In this case observe her droppings over a few days, this can give you an indication of any other organs affected by consistency and colour etc...
Temperature - check for unusual heat under wings, to identify whether there is an infection present.
Vent..does this look normal for a young pullet..should be moist and healthy looking. Check her feathering, are there any signs of moult/new feathers (also are any of your other birds going through the moult? )
Comb and face..pale or normal. Finally look at her eyes..are the pupils normal and reacting to light.
Mouth and crop - any lesions in the throat or discoloration? - crop normal tension and capacity for time of day/evening
Unexplained weight loss in chickens, especially birds recently at POL, with all the usual suspects eliminated..i.e. worms and external parasites, dysfunctional crop due to impaction (from the environment and not ill health) ..does leave you with simply she is not thriving due to an underlying problem which could be genetic or there is a virus.such as Mareks or Lymphoid Leucosis causing the weight loss.
A further examination by an avian specialist will probably be the only way to know for sure I am afraid.