we had a spate of diarrhoea in lambs last year after we had run them with our neighbours at weaning time- once we'd ruled out the obvious like worms, lush grass etc. got round to taking a sample to vet- showed coccidiosis which can happen for lots of reasons especially if lambing sheds aren't clean. bit of a faff and took a while to clear up with abs but would recommend getting the poo analysed