These are some pics, I have no clue what kind of soil it is, but I thought that since the lady at the store did it for me that it would be just fine.. I don’t know if you can really see, but the smaller pad that fell off is like lighter green in the middle and darker at the ends, is this a definite sign of rotting? and they’re not getting that much sun, it’s winter here where I live so we literally get like 8 hours of sunlight during the day and so I guess they get like 4 cuz of where the sun rises and sets isn’t seen through the window :/

Your cactus doesn't look too bad. Tbh I was expecting a mushy cactus;). Definitely etiolated. See all that lanky growth on top. That is 100% from not enough sun. If this were mine I would twist those off. They will never look right, even if you were to give more light. The new healthy growth will just grow on top of the older lanky growth. Taking those off will keep your cactus from wasting energy on weak growth.

I really do not know why those pads fell off. There doesn't seem to be rot or anything else suspicious. I definitely would still switch it out if that glass. That is just no bueno for your cactus;). Maybe your cactus is trying to tell you something by dropping those pads;). Just follow the other advice I gave you and your cactus should be fine;).

I'm sure you've encountered those little fuzzies on your cactus. If not, then consider yourself lucky. They're called glochids and are a pain.

Mine has just started doing the exact same thing, what appear to be completely healthy pads falling off. I keep it in gritty mix in terra cota and water only when very dry, I’d love to know how to fix it

Are they new pads or older pads:)?

I wonder if it has to do with light? I'm gonna do a little research:).

I think I would call mine the middle pads, the bottom ones are fine and there are some new ones starting on it as well

For the pads that have fallen, you might try cutting off the bottom third of each pad. If it looks white, clean, just let them lie of top of some potting medium, they may root out the side. I wouldn't bury any part of the pad, just let it lie on top of the soil.

Razors work well for glochids.

If the pads do fall off, you might try propagating them. If there is some rotting on a pad that has fallen off, cut out the dead part with a sharp knife (wear GLOVES for the prickles!). Opuntias are not difficult so you need to figure out what yours does not like. As above, you need to transplant out of that transparent, glass and put it into a ceramic, if you tend to overwater, use a smallish container. You soil looks peaty btw (commercially sold "cacti soils" can be terribly ill-suited). To root the pads, just rest set them on some sandy soil (they will root from either a vertical or horizontal position).

P.S., The above Opuntia is a cold-hardy one From a few pads, I now have a thick border of them.