Why am I gaining weight?

I am 16 years old, & I play softball & I'm in the marching band. I'm extremely physical. I am 5'4, and I'm not a stick. I have curves & junk in the trunk. I started out last year weighing 120- 125. Last month I weighed 130. Last week I weighed 125. Today, I weigh 150. What happened? I want to loose 20 pounds to get rid of my jelly rolls and cellulite on my thighs. How can I do so without starving myself & why have I gained so much weight?