It's not the cold so much as the shortened days.
I don't see anything too concerning with your pullet. I would get a weight on the girl you are concerned with. Then continue to monitor her. Weigh her again in 2 weeks at the same time of day and see if she is gaining, losing or holding steady...
Welcome to BYC!
How old is this bird?
What exactly are you feeding her?
How long have you had her?
Have you ever weighed her to know for certain she's lost weight?
Can you please post some pictures of her in full body profile?