u/AffectionateAct4432

Got this cactus a couple years ago from ikea, know absolutely nothing about taking care of plants i just water it whenever i remember too (not often, probably every 3 months) its been growing great but began dropping in the span of a night.
Is the pot too small? The others seem to be fine and theres no rotting that i can see in the soil or the cactus
Third picture is what it used to look like.
Any advice would be appreciated !