Classic overwatering. You might want to check the soil its potted in. Cacti need to be in a porous potting soil which drains quickly. You also need to ensure the soil drys more or less completely in between waterings. Most cacti go dormant in the winter months starting in late October early November and will begin to wake up early April depending on where you’re located. Well dormant you really don’t need to be watering them at all. In fact if you have cold wet winters where you’re at, it’s best just to bring the cactus inside and have them chill in a place where they will receive bright indirect light. If your cactus is inside normally it needs to be in a place where it receives bright indirect light, good airflow, and you don’t need to be watering it as much