One of my hens has been acting strange the past few days. She is still eating and drinking normally, but other than that she spends her entire time sitting down, looking around or sleeping with her head tucked in her feathers. Haven't seen her on the nest the past 5 days so I think she also stopped laying. She's a 1.5 years old ISA Brown in a flock of 18 and the only one doing this. She used to be very energic. The temperatures dropped last week and it's pretty cold right now. Could this be the cause?