Oddness

today (and two or three times before, but due to wet/slippy/icy conditions i ignored it) from a standing start, left foot on left pedal, i pushed down to get going. the pedal arm went round, the wheel didn't appear to. it felt very very odd. as if i was on a patch of ice, i.e. cranks spinning but no forward motion. i'm a bit confused. i can check the lockring, and am about to, but can anyone offer up some advice about what this might be? i don't think the wheel has moved forward at all. any info/advice? ta.

i looked at the patch of road i was on, no ice, no metal.