Strange desire to act like a girl, not sure if I'm a transvestite...?

So in the last two years or so, at semi-regular random points I have strong urges to dress and act like a girl, however at other points it seems completely ridiculous... Why? It's only come about in the last two years... I'm only eighteen so I can understand that it might be a hormonal imbalance... It seems to subside for long periods of time after masturbation... am I becoming a transvestite?