Is my chicken just cold?

I am new to the world of backyard chickens. I have 6 chickens. 3 barred rocks, and 3 rhode island reds. They are about 8 or 9 months old. This is their first winter. They all are healthy "happy" chickens. Minerva though, hasn't came out of the coop or even off the roost this morning. They have a nice coop. It's cold. 0 or possibly a little lower last night. Their coop does not have heat. All of my other girls are running around the run doing chicken thing, but she is just sitting in the coop. Is she OK? If it were a cold issue, wouldn't they all be cold? I'm kind of worried. I love my chickens and try to give them the best1
Thanks in advance for any advice!

 

Thank you Judy! She did finally come out. She went to the water, so I figured that was a good sign. She then went in to the nest boxes. However, when I retrieved the egg, it was a soft shell. It looked kind of messy though. Almost yolky on the outside, but the membrane seemed to be intact. Her bum looked a little messy as well A little while later, I went to check, and she's running around like all the other girls, talking to me, and hitting the food like crazy, she seems much perkier now, also, her bum looks fine and normal now. Do you think she's OK? It's not any sort of parasite or anything else that I can tell. So far as it looks, they are all healthy as can be in that aspect. They free range during the day (in the safe confinement of my almost half an acre backyard). They have a nice coop to sleep in at night. I do my best to give them the best, but this is my first "problem" in the flock.