I recently got a new cog and lockring put onto my wheel and when I ride, sometimes it feels like its jerking forward. Not the wheel itself, just maybe the cog or something twisting. I got off to look and can't budge it with my fingers or anything. Sometimes when I'm just riding, everything "clicks" forward or it happens when I skid sometimes too.
1984 Medici Pro strada running fixed, late 90's Cannondale road bike, 1999 Gary Fischer Paragon mountain bike
could be your cog is stripped.
Especially if you've got a cheaper hub, cog or both.
I don't know if you trust your bike shop, but it sounds like a bad installation to me. Have the shop that did the swap take your cog off (while you watch)
If there's any metal shavings or wear on the lockring, cog, or hub the cog is stripped and you'll need a new cog and hub.
Sounds to me like either the cog or the lockring weren't fully tightened on installation. you probably couldn't budge it with your fingers because after each time that it happens, the cog is either binding up against the hub or the lockring, but just not both like it should be. One way to check would be to push down hard on the forward pedal so that it slips forward again and then feel if you can move the lockring. However, as squintal brings up, each time this happens, your threads are more and more likely to strip, so play with it at your own risk (personally, i'd just assume this is the correct diagnosis and not ride that wheel until I could get everything tightened down again). I don't think the hub is stripped yet, though. It might be stripping, but if it was fully stripped, the cranks would just spin without you moving anywhere because there are no threads left on the hub for the cog to engage.