If they were wooden arrows and she stored her arrows outside, I would say yes, the cold would warp her arrows.
Carbon however is very resilient to cold, much like plastic, nylon and similar materials. So I would say cold would be fine.
Fibreglass arrows however are very cheap, and my understanding is that fibreglass can become brittle in very cold temperatures and develop "stress cracks". This would not change their trajectory however, what would happen is they would snap more easily - what I like to call "catastrophic arrow failure".
What is far more likely is that it was all in her head. Lack of concentration, lack of sleep, frustration, distractions, hunger, etc. can really mess with an archer's head. A mental distraction can cause a person to make minor mistakes to their form and make it near impossible to make good arrow clusters.
Or maybe she was just wearing a different jacket because it was cold outside and the jacket was rubbing against the bowstring during her releases and/or messing up her form somehow. That is probably the simplest answer.
Here is a list I made of common mistakes people make during releases that can mess up their arrow clusters.
http://www.cardiotrek.ca/2013/04/cor...y-release.html