Originally Posted by zx108

I doubt its your cog or your lock ring. If it tends to just happen randomly you probably kicked up a rock or a piece of asphalt making your cranks race without it seeming like your wheels moving/you are going anywhere. This happened to me a couple times when i first started riding fixed, threw me for a loop for a minute too.

If your uneasy just take it to your local bike shop to make sure everything is a-okay.