It's possible it could be a bit irritated due to the weather change or rubbing up against something. It's hard to tell from the photos if that is discharge/debris in the nostril, but if you are seeing any discharge, sneezing, coughing, etc it could indicate a respiratory infection as well that should be seen by your vet. Otherwise, monitoring it for any changes and keeping your cat from pawing at or scratching at it may help it to heal.