Why does Leafpool quit being a medicine cat?

Leafpool broke the warrior code by mating with Crowfeather. Apparently she did not want to tell anyone, nor risk her posistion as medicine cat, hence why she left the camp with her sister when the three were coming. They lied and said Squirrelflight had the kits, when they returned. Since no cat saw Leafpool give birth, they could not really contradict.