Could be jaagsiekte (driving sickness) a lung cancer spread by a virus. If you pick up her back legs like a wheelbarrow (the wheelbarrow test!) fluid would pour out her lungs through her nose and mouth. Known as driving sickness because they often cope fine until pushed to run and can drop dead or just wheeze and cough.
Nothing can be done to prevent or treat other than avoiding buying from farms that have had it. Keep an eye on the others and try to avoid stressing them, but any wasting away or coughing may need a vet to give symptomatic treatment to prevent/treat secondary lung infections or euthanasia if needed.
Alternatively it could be another underlying problem, such as a metabolic issue - hypoglycaemia, hypocalcaemia, hypomagnesaemia etc from lambing - or an infection, etc that she has been hiding as that's what prey animals do.
Only way to be sure would be a post mortem, but as a vet, I don't think it would be warranted as fairly likely to be an individual sheep problem so will not benefit the rest of the sheep to know how she died.
Just need to watch the others for any signs of being off feed or unwell at all, and make sure they have a decent energy/mineral lick