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There are a couple of things that may be going on and, yes, they are serious. Have you moved the bird to an isolation unit in which you can monitor her and provide needed supportive care while you sort out what is happening?
One possibility would be an egg that has broken internally - the contents may have been completely or partially expelled, but there could be shell, etc. still inside the bird. Did you examine what was expelled closely? If so, were there any shell bits with it, was it just yolk like substance or did there appear to be albumen there as well? Are you comfortable checking the bird internally - this is done by washing, gloving and lubricating your hand and then inserting a finger into the birds vent.
Did you happen to smell the expelled contents? Sometimes an ill bird will expel fecal matter that is yellow in color and it can vary in consistency - so it *could* be confused with yolk at times.