Ah, I see what it was. It was the first (and only) mod I ever installed using CurseForge. It created a directory separate from the Mods directory called "mod_data". When I removed my mods folder, I didn't notice that folder, because I wasn't expecting to see it. Once I removed that folder, everything went to normal.
Guess I'll avoid CurseForge. I downloaded that mod as a sort of experiment, and I think I'll stay away from it. I trust about a dozen modders, and generally stick to them; I'll continue to stick to those (as they update), and stay away from modders on CurseForge.
