Why Is My Phone Randomly Shutting Off and Not Charging?

I've been having issues with my phone randomly shutting down since mid-March. At first, it mainly happened when the battery was around 15%, so I brushed it off. However, four days ago the problem intensified. The screen now flashes randomly, and it restarts on its own. The battery percentage seems to fluctuate wildly; for instance, it can read 80% one moment, restart, and then drop to 20%. I suspected malware, so I did a factory reset, but that just made things worse. I've lost all my Google accounts and contacts, even though I had everything backed up. With only three days left of my break, I'm stressed out because I rely on my phone for AI assistance with my English homework, and I'm still failing. I'm using a Galaxy S22+.