I know she is gone now, but if you still have her remains- is the material hanging out of her vent, or her 'belly button' area which would be an inch or two in front of her vent- more toward her keel. Being she is that young, this could of been an infected/abscessed navel, that subsequently ruptured. Most chicks with infected navels don't live to 6wk though. If the other pullets are doing ok, no diarrhea ect- this was hopefully an isolated incident. Do check everyone's bottoms though- if there is a pecking/bully bird in there- you will see pecked bottoms in other birds soon.