What breed is she. There are two possibilities that come to mind, the first being Egg Bound and the second Red Mite.
If she is a hybrid then she is about the right age but if she is a pure breed then she is a bit young to be egg bound. I am not too experienced on this but believe that inserting some olive oil into the vent area and then putting her somewhere dark with a warm hot water bottle under her may help.
If it is red mite, they have a habit of pcking on one bird at a time. The bird will start to look anemic with a pale comb. Check inside your henhouse (paying thorough attention to you perch holders and nest box). If you see any little black, red or opaque coloured dots maximum size of a pin head then these are red mite. You may also see blood spots on the perch.
If this is the case then there are a number of effective treatments available (have a look in our store www.poultryproducts.co.uk).