Why would anyone do that?

You need to be very direct when you confront anyone who is slandering you by telling others that you said something that you did not say. If possible, meet with them face to face and look them straight in the eye when you tell them that you have been told that they have been telling a falsehood about you saying something. Tell them who told you and then set up a meeting between your Aunt and the person who told you that she was repeating an untruth. People like her need to be confronted about their lies.