My Mac suddenly ran out of space

All of the sudden my 2013 iMac on Catalina ran out of space. Last time I checked (last week), I'd about 400GB of space on my 1TB hard drive. Now the computer starts really slow and then it says I've -43Mb (yes, with a minus sign). I think the problem is related to an update (I forgot to disable updates, every update is a mess). I'd Time Machine, but my wife had to back up her Windows computer in an emergency, so I lent her my external hard drive. When the update ran, there was no external hard drive connected, and last week I saw some messages about Time Machine errors (I don't remember where, though, because I freaked out and tried everything I could ). Anyway, I deleted a lot of files, and I mean at least 100 GB. Still the same problem. Also, when I use the System Information and then Space Administrator (maybe that's not the right name, I translate from Spanish, but it's the option Cmd+U), the Documents / Large Files option is empty, although it's 177.48 GB. I'm going crazy and really don't know what to do. I've tried the software mentioned in https://apple.stackexchange.com/questions/261252/system-storage-on-macos-sierra-is-470gb to no avail, because what I delete doesn't really get deleted, it just disappears. I'm watching closely and I see it's creating files (or something) at a 30MB/s rate. So I deleted almost 2GB and a couple minutes later I ran out of space. So it seems there's some process or soemthing that's causing this behavior Does anyone have any idea what I can do or how I can find (and delete) these "ghost" files?