Need help/suggestions for my sick RIR~"feisty"..

Please give me ideas as to what to look for or to do to help my baby.The last day or 2 she is not herself.She will be 2 in April,normally very "feisty".She has no obvious deformities,she is drinking alot more than normal.She stays off to one side away from her sisters n just stands there.The other day she wouldnt come in n after shaking the treat can for 5mins,she did.Not like her,shes usually in 1st.It has been cold here,single digits,but it has not bothered her in the past.Ive tried too see if shes eating n a occassional bite or 2 of feed.I did notice her poo is all h20 n a little yellow on 1 side of the puddle of liquid she dropps.Her eyes look tired n she just doesnt seem right but doesnt appear to b in pain.All my girls r out in enclosed play area during day n locked in secure coop at night.Plenty of h20,layer feed,peeled n cored aplle slices as well as safflower seeds for treats.When the temps r too low they stay inside,even if the trap door is open,they wont go out.Is there anything I can do? Look for? check? Please help me.Thank you.