pedalling 28mm slicks on perfectly paved wet road - rear wheel slipping?

just checking what i thought i felt - new to road bikes and narrow tires. pushing probably just under 20mph on a wet cycleway seated - perfect tarmac.

i thought the rear of the bike was moving sideways, to me the rears are slipping!

i backed off and it was tracking straight again. is that normal? i'm moving to 32mm tires anyway, for more security!