[Closed] Chain skipping in smallest cog only conundrum - help needed.

Hello. Randomly in the middle of a ride the other night my chain started slipping but only in the smallest cog at the back. All my drive chain components are brand new, I changed the cassette, the chainrings and the chain just 3 weeks ago, they're allXT except the chain which is KMC. The set-up is fine, one click on the shifter takes the gears up one cog and the same back down. I checked the chain thoroughly and none of the links are stiff. I had the lock ring off and taken off and put back the top two cogs, in case they weren't fitted correctly or if the lock ring was loose. I've messed about with the b screw and in or out a little bit makes no difference it still jumps. It does it while in any of the 3 chainrings so not limited to one or the other. The only thing I can think is that I might have bashed the mech on a log perhaps but the mech on the hanger looks straight and it doesn't skip in any other cog but the smallest.
Help me Obi wan Stw, you're my only hope. I like to go fast!