your cog is slipping. this probably happened because it or the lockring was not initially tightened enough. it may have already stripped the threads - careful. don't ride it until you get it fixed; maybe you need to take it to a bike shop. or, do it yourself - remove the lockring and remove the cog. inspect the threads - make sure they look intact. if they look chewed, removed, stripped, flattened, or anything ****ed up, then you're going to need a new hub if it's not a fixed/fixed fduplex style hub.

if the threads are okay, grease the threads and put the cog back on - you can tighten it using the "rotafix" method (do a search for the link for how exactly to do it - it's using your chain as a stand-in for a chainwhip). then, install your lockring (grease the threads!) and tighten the **** out of it. then go for a ride up a hill using only forward pressure - no skidding, no backpressure. really mash down hard to make sure the cog is on as hard as it can go, then re-tighten the lockring.

it seems that a lot of factory trackhubs come from the dealers with improperly assembled cog/lockrings, and don't hold when the rider starts skipping and skidding - hence the slipping. but, if you haven't stripped the threads, there's no reason to worry - it's a quick fix.