See the post bit further up. Dogs can get blocked anal glands if they have runny poo. The solid poo normally squeezes an amount of smelly stuff out as they poo. If they have diarrhoea the glands dont get squeezed and they get too full and itch. It is a quick job for the vet to squeeze them but be aware it stinks. Once poo returns to normal it should resolve itself.
My gsd went through a phase of having them squeezed and the vet showed me how to do it but luckily the problem sorted itself before i had to try it.