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.
I'm no fan of layer feed. I don't feel it offers optimal protein levels. I prefer to offer Flock Raiser, All Flock or a non-medicated chick starter with 18-20% protein and put a container or two of oyster shell out for the active layers to supplement their calcium levels.