Fuzzy thing left by a caterpillar? are they eggs?

I found a small brown caterpillar and put it in a container with leaves and a twig (im a teenager and its summer, i have nothing better to do). So i left i there for about 5 hours annd when i came back the caterpillar was kinda twitching but pretty much dead. And there was a fuzzy thing at the top of the container that wasnt there before. Is it a nest with eggs in it? how long would it take for them to hatch?