Without being able to examine it I cannot say for sure what it is however I would suspect that it is a bruise caused by the vacuum (cephalohematoma). A bruise is caused by small blood vessels in the shin and underlying tissues being damaged and thus releasing blood into the tissues. I would expect to see some improvement by now or within the next day or so in this type of injury. I would suspect that when they said fluid they meant blood. Perhaps they think that if they said it was blood it would sound more scary than if they said fluid.