It's a CRD. As robopetz said, isolate your bird and keep her warm.
When my birds get a mild CRD, I treat with Tylan powder [which you can get from a vet.] The Tylan is mixed in the water and given for 5 to 7 days. Usually it takes 14 days for a mild CRD to run its course with a bird.
Also watch your flock to make sure no one else shows any symptoms.
Hope she gets better =)