Okay, so I had maybe 4 monsters left, but I killed Undyne, then went and killed them. Did I screw up? Right now, I'm in the Core, searching for kills. I've heard that the best place to get encounters was the bridge before Mettaton. But, I'm no longer getting any encounters there or anywhere. Also, my save points are not changing at all. They stopped telling me how many I had left after Snowdin, and previously, I've killed everything, and it showed "But nobody came.". But now, I can't even find any of those encounters! Can someone help?
Unfortunately, you should've killed every enemy before killing Undyne. You have left Genocide path and unfortunately, you are currently in the neutral path.
To successfully complete the genocide route, you always have to kill the required amount before you fight the boss. That's why I recommend farming in 1 place until the counter is exhausted. A way you could've noticed that you were no longer taking the genocide route, is that Undyne was regular, and not "Undyne the Undying", and the fight took place near the cliff, and not on the bridge.
You should've killed all the required monsters before fighting Undyne. Guess your route is messed up! Better reset, darling.