Why can't Yahoo answer my security question to my satisfaction?

Something screwy happened this morning to my Yahoo account and I have not been able to get a straight answer from them. I AM NOT HAPPY WITH YAHOO> I was being greeted by one name on Clic Mail, and greeted by another name in the NewMail format. What is scarey about this is that my daughters name, WHICH NO ONE AT YAHOO SHOULD KNOW, was being used. This happened completely out of the blue, and Yahoo has not been able to explain how.