The meaning of "afterclap" is when something disastrous occurs after the initial disaster. In the way you've used it, it's indicative of someone coming in and causing a mess or disaster when everyone thought it was over. Although, I'd say something closer to "I've/they've caused an afterclap." Jerry is right, though. No one uses this word anymore. You may hear it referencing a natural disaster (mostly commonly earthquakes) but that's about it!

This is an archaic word. There is no reason to know it. Surely someone is not teaching you this word. I had to look it up. I have never encountered it.

After an earthquake, it would always (I think) be called an aftershock.