Singlespeed & Fixed Gear - Cog or Sprocket slipping? Not too sure what's going on here?

I was riding around yesterday and I was teaching my friend how to skid on his new fixie (lol hipster) and anyways my bike started doing something weird when I would go to lock up my legs it bike would just "give" and it felt like my feet were just spinning (only for about 1/8th of a revolution though) as if the cog was going backwards on the wheel (the wheel not changing directions) I checked it out and the cog feels tight on the wheel. I would skid, but only for a tiny bit, very weird. It feels as if there is zero tension on the skid (which I know is wrong) I thought I was actually lifting my rear wheel in the air and that's why it felt so easy but that's not it. I also was climbing up a hill standing up and it kind of "dropped" me or atleast it felt that way. Any ideas what this could be, or what I should check out?

The bike is pretty damn new, so I'm not too sure what's up- I bought this as a "fixie" after using a converted one I made myself for two years. (the one I gave to my friend)

The bike is a 2009 Globe Roll.

Thanks.

If it happens both when you start from a stop and when you skid, then it's most likely that the cog is slipping. Get a chainwhip and a lockring tool and give it a good tightening. It's a good idea to do this once in a while if skidding/locking up the rear is your main method of stopping.

LOL I had the same problem just couple hours ago on the TK2. Turns out my lockring was just a tad loose so after it was tightened, it was all good to go. That is most likely your problem.

yeah it happens. Tighten that **** down.