If she is painfully thin I would feed her (and only her and her lambs if they want to, not the whole flock) a proper ewe&lamb mix for a while. It will help her building up her reserves again, shreds is good but you will need a bit more.
Usually a single shot of Calciject is enough, but be prepared to repeat if necessary. I would also drench her with twin lamb disease stuff, or Cetophyton.
I have heard that there is a new product called EWE-GO or something like that available which has the calcium as part of the drench, so no injecting necessary. BUt haven't been able to speak to vet yet to see if he can get it for me. (My goats can go down with milk fever sometimes, and they are a nightmare too inject compared to sheep.)