BB unthreading?
 

I was riding down my driveway today, and as I was slowing down my pedals "slipped" backwards 2 or 3 inches. In other words, as I was pedaling backwards to stop, they jumped backwards with no resistance. The chain did not skip, I am positive. I then pedaled forward very hard and the same thing happened again, in the other direction, but after that I could not get it to skip forward again, so I assume I was untightening the BB as I braked, but re-tightened it when I pedaled forward? What is happening and how can I fix it?

Thanks guys

Maybe your lockring isn't cranked down enough?

Your cog is slipping. Get to a shop asap if you don't know how to fix it.

Ah, thanks, didnt even think about the cog, I guess I didn't get the lockring tight enough. Thanks for the help