Xbox Live won't let my avatar try on girls clothes because it thinks Im a guy (I'm a girl). How do I fix this?

I've always wondered why they never gave me girls clothes to choose from. Today I went on the website because I was thinking about purchasing some of the avatar clothes. When I went to the dressing room to try them on of girls clothing it says "Ooops! This item is for the opposite gender." My avatar looks like a girl, and I dont ever remember setting anything to "Male". How can I fix this?