Johnson grass is a relative of sudan and can have nitrates. However, death from nitrates occurs within a matter of minutes of ingestion. They don't lay around for a day or two.If you looked into her mouth you may find she has little or no teeth left. Sometimes a cow can look perfectly normal up until their end. They go downhill fast when their teeth go. Another and even more likely cause is poisoning through whatever she may have ate.
I've seen a cow die within hours of eating Johnson grass. It's highly toxic to them but they will go after it sometimes. I would poke around their grazing areas and make sure there aren't any big patches of it around. Golden rod will eventually give them the trembles and kill them too.