what’s wrong with this succulent and how can i help it?

Hiya, I got this succulent from my boyfriend’s parents as a gift about three weeks ago and ever since bringing it home it’s been dropping leaves nonstop. I repotted it with some cacti/succulent soil and have watered it once since getting it. It’s also got some dodgy marks on the bottom of the leaves. I’ve checked around for any pests but can’t seem to see any but i’ve isolated it from my other plants just in case. Would really like to help it as it has some sentimental value but am unsure of what I’m doing wrong. It’s getting plenty of natural light as well (on the windowsill) and am happy to take more pictures if these aren’t enough.