Closed "Client-server connection is no longer valid" SINGLE PLAYER

I'm not sure why I got kicked from my game when I was running in single-player, but it happened.

I was idle for a little bit in-game - I'd alt-tabbed over to another program. When I alt-tabbed back, I started to move and immediately got kicked to the main menu with the notice "Client-server connection is no longer valid." There was no other error code or anything of that sort.

I can confirm, also, that my internet connection is stable and wasn't stuttering or anything at the time - I was also on Skype, and chatting on another site. I also have a constant ping running in a console window due to the fact that sometimes my internet DOES stutter (but it wasn't at the time of this error - ping was about 23~24ms).