Automatically switched from my single player world to a multiplayer server

I was playing in a single player world I had created to do redstone experiments. No mods, vanilla minecraft. Prior to this I had been playing on a multiplayer server: play.phanaticmc.com. All of a sudden, I was switched back to the server from my single player world and I was falling slowly, like the chunks hadn't loaded. I exited the server and rejoined my world, and then I got the Minecraft is out of memory error. I was eventually able to rejoin my world.