Hard drive free space disappearing rapidly for reasons I don't understand...

First uninstall CleanMyMac by following the developer's instructions. Anti-virus apps, cleaning apps, and third party security software is not needed on a Mac and usually causes more problems than they solve plus they impact system performance.

You may have installed some malware so I suggest you run MalwareBytes to check. I know I just said these apps are not needed on a Mac, but when your Mac is acting like this I would be concerned about some malware being on the system so it is best to check just to be sure. You can uninstall MalwareBytes after running the scan (the uninstaller is located on one of the app's menus IIRC). This is the only anti-virus app that actually works on a Mac and it is the least invasive of those types of app. Normally even this app is not needed if you practice safe computing habits.

If you have trouble running MalwareBytes, then boot into Safe Mode to install and run MalwareBytes. In theory booting into Safe Mode should stop whatever is using up all your free storage space since Safe Mode disables all third party drivers and even Login Items.

To look for other software issues run EtreCheck and post the report here using the "Additional Text" icon which looks like a piece of paper.

It sounds like you had some app that was just running out of control. Your computer can actually use the hard drive for additional RAM. In rare cases, an app can spiral out of control and try to use 100 GB of RAM. If that happens, it can consume 100 GB of storage space because few people have that much RAM.

It can be difficult (although not impossible) to go back in time and find out what app was responsible.

Hi there Nate Harrison,

Welcome to the Apple Support Communities. It appears that you'd like some information on freeing up storage space on your MacBook Pro, so we'd like to share a resource with you. How to free up storage space on your Mac will walk you through some steps that will help with managing your device storage.

Take care.

I'm glad you were able to locate the problem.

The battery condition is listed as "Service Battery" which indicates macOS has detected a hardware fault with your battery. I would recommend having Apple replace the battery for you especially while you can still get an official Apple battery. The quality of third party Lithium-ion batteries is extremely poor even when purchased from a reputable vendor.

You definitely want to check to see if your laptop qualifies for this Apple Battery Recall:

https://support.apple.com/15-inch-macbook-pro-battery-recall

If your laptop qualifies for the Battery Recall you need to have Apple or an Apple Authorized Service Provider replace the battery for you since the Battery Recall is meant to address a safety issue with the battery catching fire. If the laptop qualifies for the Battery Recall the battery replacement will be free. Only certain laptops are affected by this safety issue and are identified by serial number.

Duplication of files or hacked or fake device