I'm not very experienced so don't take my word for gospel, but....
- any sign of persisting worms or other illness? Fluke? Does the wormer you used cover all kinds?
- is she lame? Can she physically get to the food, water?
- if not, and she's just losing condition from being in lamb, then I would carefully add food for her. Yes the risk is the lamb gets too big; but on the other hand a starved - or worse, dead - ewe won't be much good for lambing/mothering

HTH