Has she been wormed post-kidding? My first guess would be that nematodes need treating, I have used Oramec this spring successfully. Use any oral wormer at 1.5 to 1.8 of the sheep dose, so quite abit more. If you are not sure a quick (if you can get it quick) worm count may be best.
Do you feed any milk replacer in top of her milk - I would think a litre per kid per day is not enough for 5 weeks of age. What breed is she?