OK...help!!! I'm thirteen years old?

Ive been cross-dressing extensively in private the past couple of months, with my sisters clothes. Panties, bras, makeup, heels, tights, ive been going all-out. No one knows I do this, nor do I wish to let anyone know. I'm just not sure if it's a or not. I'm certainly not gay, nor do I feel like a woman trapped in a mans body. I heard that a cross-dressing is just lingerie and tights and stuff. I like looking pretty doing it, wearing makeup and such. I really don't know why I like doing it. I play sports and never thought about changing es, I just like looking like a girl occionally. What do you guys think that the problem might be, and is it just a phase?