My understanding is that with true Twin Lamb Disease, once the lambs have been expelled, the ewe is pretty much instantly recovered.
So I'd go along with the worms, general poor condition (it's been one helluva year for them all), or poss incipient milk fever.
Besides what others have suggested, a jag of Combi-Vit wouldn't hurt.
What about fluke? Lots of farmers have lost lots of ewes to fluke this year - fluke where it's not usually; fluke in seasons when it's usually absent; fluke even after flukicide given. Loose poo, undiminished appetite - could be fluke. Are they thin? A bit puffy under the lower jaw? If it's fluke, they'll be thin and could have the puffy lower jaw (but absence of puffy lower jaw does not mean it isn't fluke.)