Could she have eaten something metal that ended up in the run? Heavy metal poisoning also seems to have similar symptoms. There's some info here: http://www.avianweb.com/heavymetalpoisoningbirds.html
I am so so sorry for your loss.
Has there been any fertiliser broadcast near you recently? The only thing I can think of off the top of my head that causes the thirst followed by convulsions and death is some sort of agri-chem poisoning: pesticide, herbicide or fertiliser.
The "lots of...