I've got 13, 6 week old chicks... we hatched them.. they were on medicated feed for a month then moved to flock raiser.. they were doing fine... then yesterday, 1 of them got lethargic and died.. in an hour.... woke up this morning and another 1 dead.... we've raised chickens for years and never had this happen... we've had an occasional bird die but this seems to be something serious.... any ideas...