My guess would be underwatering, but what I really want to know is.....is that a plastic drinking cup???

Rachael- Here is what I would do:

1) Replant the cactus in a terracotta pot. I'ts much healthier for the cactus because, it's a natural material. it has better aeration and will dry out faster than the plastic cup you have it in. It will also have a drainage hole. Note: plastic such as your cup will retain moisture. That is not good for any cacti or succulent. Once it's roots sit in a moist/humid environment for too long, the plant will start to rot. Which, leads us to the second step...

2) From your picture, the soil looks dry as sand all the way to the bottom. When was the last time you watered? Some cacti especially ones that have "ribbing" shrink and expand depending on how much water they've retained or lost. That in itself is normal. Your cactus is probably just thirsty. -- Soil: You need to add perlite or pumice, something gritty to make the soil drain faster and be more breathable for the plant. the easiest to mix: 1 part cactus soil and 1 part perlite, then mix well. Keep doing that..at that ratio until you have mixed enough soil.

3) The soil should now be mixed well and ready for your plant. Place the cactus in the soil at the same level it was in as in your photo.

4) Wait a day before watering. Then, water the cactus until the soil is saturated and the water starts to drain out of the bottom of the pot. Once the water stops dripping, you can water it the same way again, just to make sure the soil is completely wet. Let the water drain again and do not water until 7 or 10 days have passed. Water once a month in the winter season.

5) place the cactus in a bright/sunny location, establish a good watering schedule (every 7-10 days) and you should be good to go.

@HU-551201314 You should start your own discussion rather than gravebumping old threads.