well knowing nothing about sheep, but at the risk of sounding rude, and I don't mean this that way at all, did you have a vet look at her? You said she had an infection that was going away, but how do you know it was going away or that she didn't develop something more while she was sick? I had a dog that got sick and the vet treated him for a sinus infection, and he got better, but it was just a smokescreen caused by the meds. Once he was done w the meds he became sick again and the vet did tests and discovered his real problem was fluid around his enlarged heart
so sometimes, an infection can mask something else or vice versa. In my experience an animal with babies usually has a 'will' to live, so I think it unusual for her to let herself go without trying since she had little ones. You don't say how old she is, not sure how old sheep get to be, but maybe it was her time if that was the case. So sorry this happened to your sheep