Although we're on a cattle ranch here in the western U.S., neighbors raise sheep. As a boy I also had experience with lambs for a 4-H project. Through these I became familiar with the axiom that there is no such thing as a sick sheep... meaning they are either well or dead. Having said that, I did have "bum" lambs (lambs discarded by their mothers) that died as you described. They were found to have experienced "Overeaters Disease"... a common phrase describing this:
Type C Enterotoxemia: This is a disease of young suckling lambs or bottle fed lambs. The causative agent is a bacterium, Clostridium perfringes Type C. C. perfringes is an anaerobic bacteria that resides in soil and manure and forms spores that are highly resistant to disinfectant and environmental breakdown. Essentially C. perfringes is present in all types of soil throughout the world. C. perfringes Type C causes death in young suckling lambs. The bacteria enters the body through the mouth either from contamination on the udder of the ewe or from suckling on dirty wool or manure tags in effort to find the teat. The bacteria grow rapidly and produce a toxin, which causes rapid death. The lambs are rarely observed ill but rather are simply found dead in the morning. If observed before death treatment is unsuccessful.
Again, echoing the previous, there's little or nothing you could have done about it...