You sound as if you have a very sick sheep! Problem is just looking at them is often misleading as being woolly they look ok even if very thin! Are you handling the sheep regularly so that you know what their condition is like? They do not get that thin overnight!
When did you last worm them and what with? Do you rotate the pasture?.....I know you don't have a lot at the moment!
Have they been given fluke treatment?
If not scouring but thin and hunched and poor I would personally suspect some sort of parasite such as fluke, however if scouring, could be worms but scouring not always a sign of worms! Could be a clostridial disease....but scouring often present....
Its very difficult to diagnose what is wrong with a sheep without a lot of history of management, medication, vaccination, feed regieme etc but the usual thing at this time of year, if pasture is damp or near water, if sheep thin, poor and ill, if all other dosing and management ok....... I'd suspect fluke.....