Mature winter bees of the hive will take cleansing flights on days when the weather will allow. If is warm enough I have noticed they will get out and forage as you found your bees were successful in bringing in pollen just before the weather made the hard turn downward. December is the time of the lowest level of brood in my area. That has a great influence of why many treat with OAV in December, getting the most bang for the buck so to speak, for mite killing when brood is at its lowest levels and numbers of the year.
As for you noticing winter pollen, I had a youtube pop us notice that David at Barnyard Bees posted a video of 16 hours ago titled "Where does all the winter pollen come from". I have not watched the video, (yet), to see what his thoughts are about it.
Phillip