Sheep can get the skits if they eat too much so it might just be that, check they're not getting to some plant or other they might not like (laurel, shiny leaved plants) or something that has blown in from a neighbouring area. I wouldn't act too soon, if they look ok I'd give them 24 hours, if they look poorly, dull eyed then it's more serious.
I have read on here that some shepherds give kaolin & morphine (from your local pharmacy) just to tighten them up a bit, but I reckon it's always best to skit it out rather than keep the bugs in the system. Let us know how you go C and well done on spelling diarrhoea, would have had to look that one up myself...