Certified GKH Gardening Expert
Looking at these photos, I suspect your cactus is not getting enough sunlight. You may want to give them some artificial light over the winter.
It could be a watering problem, but they tend to have the same response to overwatering as they do to underwatering.
Here's some info on watering: https://www.gardeningknowhow.com/ornamental/cacti-succulents/scgen/watering-cactus-plants.htm
Their native habitat is the desert, so if they aren't under desert conditions, they can become stressed. Check them for spider mites, as well.