update; The condition is called Hydramnios and is caused by the foetus being defective, in that it has a defective swallow reflex and therefore doesn't re-ingest the fluids so that they can be expelled by the ewe. Basically, the foetus pees, swallows it and it goes back into the ewe's system via the placenta. It's not so commonplace and there is no reason to suppose it will happen again - it's just one of those things. The foster lamb grew fast on her milk and is doing well.