Cactus Deflated??? Help!

I have 3 tiny cacti (1-4 in. high). They are all planted in the same type of pot and soil and sit in the same location, and are all on the same watering schedule. I have had the cacti for about 3 months with no issues. Suddenly, yesterday, I noticed that 1 of the 3 cacti is totally deflated...as if there was air filling it and now the air has been released. Any ideas on why something like this would happen? I am extremely concerned that whatever happened will also happen to other 2 cacti since I treat them exactly the same. I doubt that this cactus can survive, but I want to prevent it from happening to the other 2...Please HELP!!!

This pic is what my cactus looks like now...

This is what the cactus looked like when I first bought it (the one on the right)...

How much have you been watering? And does your container have a drainage hole? I sort of suspect a watering problem, but I don't grow a lot of cacti so I'm sure there are other possibilities as well.

I would suspect rot. It would be caused by too much watering/not enough sunlight.

I water about once every 3 weeks and, no there isn't a drainage hole. I will take the other two cacti out and put drainage holes in their pots.

I bet that's what happened, even if you're only watering infrequently if the excess doesn't have anywhere to go it becomes very easy to overwater even something that's more water tolerant than a cactus. The only plants that you can safely grow in containers without drainage holes are bog/water plants or plants that like super moist soil, anything else you need to have holes in the bottom. If you have a pretty pot that you want to put the plant in but it doesn't have holes and you can't easily drill some, you can put your plant in an ugly pot with a drainage hole, then stick that inside the pretty pot. When it's time to water, take it out of the pretty pot and water it, then once water's done draining through plop it back in. That way you can have the best of both worlds.

Okay, so here's an update on my cacti. About a week after my first post I noticed that one of the other cacti was starting to deflate as well :( so I decided to re-pot and check for root rot. The roots on all three looked normal and healthy and the soil was dry all the way through even without any drainage holes. I did drill a drainage hole and remove the rocks as ecrane suggested as a matter of precaution for the future. I re potted all three and sat them closer to the window, taking heed to jolene's suspision that the cacti may not be receiving enough sunlight. The cacti have been in their new location for about a week now and I must say that I can feel the difference. Now when I gently squeeze or apply slight pressure to the two that were deflating, they no longer "mush in". It's like they're becoming stronger from the inside. I didn't think the little white one would survive, but now I do have hope because things seem to be going better. Thanks again for all of you guys' advice! I will keep you posted on the progress of the plants. :)

SGT

Glad to hear things are looking up for you cacti! Sounds like you've got them in conditions that will make them much happier now so hopefully they'll continue to improve!