Difficult to give a definitive answer without seeing it but it could be a burst water bag, in which case she's started to lamb. In a first timer vulva and udder may hardly change before birth. Is she pawing the ground, lying down then getting up again, tucked into a corner or under a hayrack, lying down and stretching out her legs, bleating, making nibbling movements ....? Depending on breed you may be able to see two distinct "dips" in front of her pelvis if you look along her sides - this indicates the lambs have moved into position ready for birth.