If it were EYP then likely antibiotics would be needed, I don't think it likely that the infection would clear up on it's own. If it only happened once, and cleared up I would be more inclined to suspect a glitch in the system, which can be caused by all sorts of things like stress (which encompasses a pretty broad range of causes), going in and out of lay, etc. Sometimes that happens and the cause is never known, if it's a one time thing then it may not be anything to worry about. If she seems fine now, I would just keep an eye on her and if you see any signs of illness then follow up right away.