good afternoon,,
leave us not go into undue worrying,,
chickens die,, mostly from undiagnosable problems,,
your chicken seemed to go through a normal bodily functional shut down,, people do the same thing..
If a lab has a problem putting their finger on a cause, how does anyone without any medical training expect to ..
I would rule out bird flue , in this case..
to me it sounds like a digestional tract malfunction,,
If the bird was a commercial egg layer type, this type of death is normal..
IMHO
I doubt it is anything contagious,, just watch the rest of the chicks for a few days and if nothing turns up, forget it and move on..